Knuck Return Lines

Much as I love the fact that I am able to make a living (of sorts) working on Harley motors, in every profession there are inevitably some aspects of the job that we don't look forward to performing.  Sometimes there is not even a rational reason for the mild distaste that the prospect of performing the operation produces, which often leads to putting it off as long as practical.

Boring cylinders has long been one of those operations for me.  Totally irrational, I know; I have equipment that does an accurate job, and good measuring equipment that confirms that accuracy.  The fact is, my shop would probably do better financially if I spent all day, every day, boring and finish honing cylinders, yet I am thankful that the volume of cylinder boring that comes through my shop is not that high.

But then there is the other type of job that I enjoy even less.  The kind that is time consuming and difficult to produce good results, which in turn leads to being a losing proposition from a financial standpoint.  However, sometimes after years of "fighting" an operation which fits that description, you come up with a method that takes some of the pain out of it.  One of those dread jobs is what I'd like to present here.

If you've been around Knuckleheads for a reasonable length of time, you have probably noticed that some of the small oil lines on the heads which connect the lower spring covers to the back side of the rocker boxes, don't always age gracefully.  Oft times the flare that provides a seal to the rocker box is the first to cause problems, whether it be split from often over tightening, or a case of just one too many on/off repetitions.  Then there are those lines which have been in contact with parts they were not designed to share such close quarters with, resulting in spots nearly to the point of striking oil.  And lets not forget rust, because it never sleeps, and eventually that tenacity will likewise make a tube unfit for service.

A twofer: at the top center of pic you can see the tube is flattened out, and at bottom left the flare fitting has a previous emergency repair via a compression fitting

What to do?  Replace the whole assembly?  While that might be tempting, once you determine that these particular parts seem to only be sold in sets of four, and then price them out, ...ouch.  The logical thing, and what we were forced to do for many years when there were none being reproduced, is to replace just the tube.  Now, it may very well be that someone somewhere is reproducing and selling these tubes, but if they are I have not been made aware of the fact.  Instead, for as long as I can remember, I have had to fabricate my own replacement tubes to braze into the lower spring cover.  And it has always been one of those jobs that I dread, because replicating all of those complex bends never seems to go well.

For years I contemplated building a fixture for making all the appropriate bends, or rather four fixtures since each cover takes a different length and shape.  But each time I considered it, I concluded that I lacked the time to undertake such a project. Instead I would painstakingly bend each tube a little at a time trying to duplicate the original's shape.  If you have ever attempted it, you know how frustrating it can be.  The first bend was usually the only one that one could get right, from there it was a tale of bend, hold the new and the old side by side to see how well they matched, bend some more, match up, try to unbend that last one that didn't match up so well, and so on.

Here is a method that I have found to be much less frustrating, and best of all requires a minimal investment in time and money to get started.  First is the purchase of a brake and tube bending pliers.  The one pictured was under $20.  If you have previously had to fabricate your own line, you probably already own something similar.  The only other piece required is a piece of scrap aluminum thick enough to take 1/8 pipe threads and long enough to clamp into a vise.

A few tools that can help make a tough job a little easier

The fixture shown in the upper left is just a plate with two holes drilled and tapped for the same fitting as is used in the rear of the rocker box.  You want to put them as close together as possible while still allowing room to tighten the flare nuts.  Now, in the picture you may notice a couple of additional items that I fabbed up to make the process yet a little less painful.  They are merely a couple of clamps made from scrap aluminum plate by drilling two holes (the diameter of the tubing) which are the same distance center to center as fittings in the fixture.  Then cross drill a pilot hole which will be tapped on one side after you saw the plate through the center of the clamping holes.  The clamps are not essential, but I found them to be quite helpful.

To fabricate a new line, your first step is to cut a piece of tubing to a manageable length and then flare one end of it.  Note that these fittings take a single flare, not the double flare used in most brake line applications.  Once you have a nice flare on a new piece of tubing, mount it to the fixture with the original drain tube next to it, and with the fixture in a vise, commence the bending process.

With the lines side by side all that is required in your bending is to keep them running parallel

The additional clamps make it easier to keep the lines parallel to each other

Once you have duplicated the original line, all that is left is to braze it back into the lower spring cover.  It is a good idea to mock it up with the head and rocker box  in order to get just the right orientation.  Hope you haven't painted your heads yet!  One other quick tip before I go; the lower spring covers are very prone to cracking just below where the oil return line is attached as shown below:

The only thing worse than a cracked lower spring cover that goes unnoticed ...

... is four of them that slip by.

This last tip about the cracks seems obvious once it has been pointed out, but I can't tell you how many times I have had customers bring in nicely cleaned, straightened, and Parkerized covers that still had cracks that gone unobserved simply because they did not know to look for them.

Valve Stem Protrusion, Part 2: The Knucklehead

　
Now that you have been introduced to the basics of valve stem protrusion specification and how they pertain to Panheads and Shovelheads, let's go back to the beginning, so to speak, and look at what most would concede to be Harley's first modern OHV engine, the Knucklehead.

Time to "fess up." The question of the valve stem protrusion specification for a Knucklehead is one I have never been able to come up with a definitive answer to, and I have been pondering it for about 30 years. Apparently the factory did not feel it was important enough to publish (at least anywhere I have been able to find) or just overlooked it.

One of the figures I've seen offered (it was on the Internet, so it must be true!) is that the factory spec was 1.575-1.580" measuring from the shoulder on the guide to the tip of the valve. While this may be correct, no source was cited. Did it come from a factory drawing or was it info passed on from someone's step-uncle who once heard it from a guy who knew someone who worked on Harleys a lot under a shade tree in the back yard to supplement the income from his day job changing tires and doing oil changes at the local Texaco station?  Either way, this spec, to be accurate, would need to take into account the differences in the thickness between stock and reproduction gaskets that go under the lower spring cup. I cannot be sure what the OEM gasket thickness was, but from supplier to supplier I have seen as much as .030" difference in the thickness. Then if you use 2 gaskets under the intakes as the manual calls for, you have doubled any error. Incidentally, if the factory was able to hold a .005" tolerance on valve stem protrusion in the '30s and '40s, they were doing far better than what is seen from them today (admittedly a very real possibility).

Another of the "specs" I have seen given (also on the Internet, so it also must be true!) for Knuck stem protrusion is 15/16" above the top of the valve guide for the exhaust and 7/8" for the intake. Even if a source for that info was given, it would be meaningless without the dimensions of the OEM guide top.  The guides available today are not even the same shape as the originals above the flange, let alone the same height.  True as it may have been at one time, it is about as useful as giving directions to a stranger that include the phrase "turn left where farmer Smith's big red barn used to be."

One other "spec" I have seen published in a recent book, also without a source quoted, is intake 1.525" minimum/1.570" maximum and exhaust 1.575" minimum/1.620" maximum. That would also suffer from being at the mercy of gasket thickness.

I also tend to think that any stem protrusion "spec" that is given with intake and exhaust being different one from the other is probably suspect, because that would mean that without shimming, the spring installed height and seat pressure would be different intake to exhaust. I am quite sure valve spring shims were not factory installed.  My guess is that those "specs" were extrapolated due to the difference in overall valve length on Knuckles, intake verses exhaust.  Either that, or valve seat pressure difference intake to exhaust was considered acceptable by the factory.  The difference of .050" of installed height would work out to about 16 pounds more seat pressure on the intakes if KPMI's stock replacement springs are indeed made to exact OEM spec as advertised.  That is probably within the realm of possibility also, but again I would like to see a source.

And speaking of extrapolation, or my best guess if you prefer, here is my theory:

There is a chart on page 82 of the Panhead Service Manual 1948-1957 Rigid that lists specs for 18204-36 inner and 18203-36 outer OHV springs. Those would be the stock springs for a Knucklehead.  The chart lists a compressed length of 1.40625" for the outer spring under a column labeled "valve closed" along with a "valve open" length of 1.0625".  That leads me to believe that the 1.406 figure would be within the acceptable range for spring installed height. Adding the lift of a stock cam, which the same manual lists as .343" for a Knuckle on page 89, to the open height of 1.0625" gives 1.4055" which matches the 1.40625" after allowing for rounding up or down.  Now, the question becomes, does this spring installed height reflect a minimum or maximum valve stem protrusion, or is it somewhere in the middle?

Back to that table of specs.  According to it, the "Knuck" springs (we'll call them -36 springs from here on out) were used on all overhead valve engines (read: Knuck and Pan) except FLH (the higher compression 74"). That reveals a bit of a discrepancy hidden in the specifications.  Since the same valves, collars and keepers were used for both FL and FLH Panheads, it is safe to say that the -36 springs could safely be installed at the same height as the stronger FLH spring set, especially given the fact that there is no separate stem protrusion spec given for Panheads, FL versus FLH.  But the "valve closed" spring height for the FLH spring is 1.375 rather than the 1.406 of the -36 spring.  That would put the acceptable spring installed height of both spring sets somewhere between those two figures.

Interestingly, if we give the 1.375 figure a +/- tolerance of .005" we come up with exactly the 1.370" to 1.380" recommended installed height for KPMI's "stock replacement" Knucklehead spring sets, which are claimed to be "manufactured to exact O.E.M specifications."

Now it happens that I have measured enough Knuck valves, springs and collars over the years to confidently say that the spring installed height on a Knuckle will be .200" less than the valve stem protrusion measured from the tip to the top of the lip on the guide.  That .200" figure is valid when using stock spring collars and valves with stock keeper groove location.  It is also subject to the slight variation resulting from manufacturing tolerances.  That means if we were to take the 1.370" spring installed height and add .200" to it, we could safely extrapolate (there's that word again) a 1.570" minimum valve stem protrusion.  If we then want to take a stab at a maximum stem protrusion, I would suggest the same .045" spread that the Pans and Shovels use, resulting in a maximum spec of 1.615". 

But remember now, while that may provide a stock stem protrusion spec (depending on whether you buy into the multiple assumptions and extrapolations which I have made or not), this will only get you to the correct valve spring installed height.   One still needs to take into account the discrepancy in thickness of the gaskets under the spring cups if you are really concerned with getting the valve to exactly the same relationship to the head and rocker arms as when it left the factory.  Back here in the real world, though, the effects of the gasket thickness on rocker geometry are minimal.

In fact, I would go back to what I presented in the previous stem protrusion post about how changes in valve lift affect theoretically correct rocker arm geometry. If you are running a stock lift cam, then you are done. As I mentioned, according to the Harley's service manuals, the lift of a Knuckle cam is .343".  If anyone is wondering about this seemingly odd number, just convert that figure into a fraction and you will see it was quite obviously a rounding off of 11/32" - over the years Harley has been very comfortable with the use of fractions in their design parameters.   Incidentally, if you notice that I have been playing fast and loose with the terms cam lift/valve lift here when writing about Knuckles, its because the 1:1 rocker ratio actually makes those figures interchangeable, unlike other OHV Big Twins.

But if you simply read cam manufacturer's literature you might not realize that the stock lift was only .343".  The Andrews "S" grind, which is advertised as a stock replacement for restorations is .355" lift.  Lieneweber's mildest Knuck cam, the "0", is still a bolt-in but has .365" lift.  Back when Sifton was Sifton, their only Knuck grind came in at a whopping .450" lift.  Our friends in Viola seem to be the only manufacturer who makes a true stock replacement grind (even if it is listed as .346" lift). 

Using my "theoretically correct rocker geometry" method of adding 1/2 the increase in valve lift over stock to the minimum and maximum stem protrusion specs, one can see that even the mild bolt in cams add a little wiggle room.  The 110 Sifton on the other hand, coming in at .106" more lift than stock adds a full .053" to the specs.  In other words, if you accept my extrapolation (had to get that word in one more time) your stem protrusion specs would go from 1.570" minimum/1.615" maximum, all the way to 1.623" minimum/1.668" maximum.  That would, of course, create other issues that would need to be addressed, such as spring pressure and installed height, not to mention top collar to rocker arm and cover clearances.  But all of that will need to wait for another post.

One last point.  the figures I present here are my best estimate based on the numbers we do find in various factory service manuals.  I will be happy to print a correction if someone can provide their source material for conflicting numbers, or provide reasoning that improves on my own.  By no means do I mean any sort of insult to those who have presented different figures.  It is entirely possible that the whole concept of minimum and maximum stem protrusion specs was not even on the factory's radar before the printing of the Panhead service manual, and was merely left to the common sense and/or whims of the re-builder.

And in case anyone is not yet sure, the word of the day was, as you probably guessed, "extrapolation."

Valve Stem Protrusion; Knucks, Pans, and Shovels

Harleys are very rebuildable, and I would go so far as to venture that they may be among the most commonly rebuilt (using the term "rebuilt"somewhat loosely) of any engine family in existence. Such a supposition is somewhat bold, given the minuscule number of Harleys compared to the vast oceans of, say, small block Chevys. But face it, which engine is more likely to wind up in a scrap yard when it is in need of major repair?

Given that, along with the often less than spectacular life span of a top end rebuild on Knuckles, Pans, Shovels and Sportsters, many if not most have seen multiple valve jobs over the decades. Naturally with each valve job performed, the valves will seat a little deeper in the head. The method of gauging how much deeper is via the valve stem protrusion specification. Valve stem protrusion is one of those specs that is sometimes overlooked and to some extent misunderstood when dealing with Harley heads.

At issue are a several things. In no particular order; valve spring installed height, shrouding of the valve in the chamber, compression ratio, and finally rocker arm geometry. Having less than the minimum can lead to the devastating result of your valve springs reaching coil bind while your cam is still trying to lift the valves higher. Not a good situation and can usually be summed up as 'broken parts."  On the opposite end of the spectrum is the Panhead that looks as though the pan covers have been bashed out with a ball peen hammer (because they have indeed been bashed out with a ball peen hammer) so that the valve spring collars would not hit them.

Shrouding of the valve in the chamber from the valve being too deep is fairly easily remedied by a judicious modification of the chamber during the process of a valve job, though this too can overdone resulting in issues down the road when new seats are installed.  Along with deep valve seats comes a reduction in compression ratio (aggravated via de-shrouding) by making the chamber larger.  That may or may not be an issue depending on a number of factors.

Valve train geometry is also at issue, but I will attempt to address that later in the post.

To examine this subject I would like to start in the middle and work our way forward in time before going back to the beginning - that beginning being the Knucklehead.

On page 75 of the Harley Davidson Panhead Service Manual - 1948-1957 Rigid, we find what seems to be first official mention of the specification (at least that I can find).

The spec, which the drawing refers to as "Valve Seat Tolerance" is pretty self explanatory. It is the distance from the tip of the valve stem to top surface of the collar of the valve guide. The illustration also shows a gauge which was available for those lacking precise measuring tools or for quick checks. The gauge is simply a cylinder that straddles the guide. The "step" at the top of the gauge indicates minimum and maximum height; if the tip of the stem falls between the top and bottom of the notch, the stem protrusion is within spec.

The 1978-1/2 to 1984 FL/FX 1200/1340 4 Speed Service Manual (note the title may not be growing in length but it certainly is in use of numbers) shows the same illustration (page 3-18) for 1979 and earlier, but it might be worth noting that it offers a different illustration and spec for 1980 and later.

The difference, at least in part, is due to the changeover to valve guide seals. Earlier heads, both Pan and Shovel, only required a machined pad that was at least the diameter of the valve guide collar to locate the guide since the lower spring collar rested on the collar of the guide. The addition of seals made it necessary to rest the lower spring collar directly on the head to provide room for the seal, so the machined portion of the spring pocket was increased to the diameter of the lower spring collar.

Late vs Early

At first glance one might assume that the different spec is due to taking the measurement to a different surface, since it is now from the tip of the valve to the surface that the bottom of the guide collar seats against. And maybe that's the case, however, things don't seem to quite add up. If the collar on the guide is nominally .100" thick, then all is well. Add .100" to the early 1.500" to 1.545" spec and you come up with the '80 and later spec of 1.600 to 1.645". Ignoring the '80-'81 guides that used a .075" snap ring instead of having a guide with an integral collar, there is still the question of the gaskets that were under the guide collar on earlier motors. I had to look pretty close to even find the part number (18196-51) for this gasket in a Harley parts book since it does not appear in any later copies, though I have a small collection of them left over from top end kits. Measuring a random sample of these showed that they ranged in thickness from about .030" to .040". The James Gaskets catalog lists them as .031" thick with the application being 1951 to 1978.

Hmmm,... so with a window of only .045" in minimum and maximum stem protrusion, we find a variance of at least .030" just in whether or not a gasket was installed under the guide when rebuilding. And what about '48 to '50 Pans and '79 Shovels? Won't they show up as nearly at maximum protrusion right from the factory? And what does that mean when considering '80 and up which certainly never used the gasket? Now the .100" difference in stem protrusion spec doesn't add up so neatly because you have an "effective" guide collar thickness of .130" (collar + gasket) for many years.

Add all of this together and I think its safe to conclude that stem protrusion specification is probably not something will "make or break" your valve job unless you wander too far afield. My guess is that the spec was added after the fact as a guideline for mechanics rather than a part of the original design parameters of the Motor Company.

And if all doesn't throw enough margin of error into the equation, then consider this. If the Motor Company's stem protrusion specs theoretically provide correct valve train geometry (and that is a gigantic stretch given shops such as Baisley High Performance have presumably made a fair chunk of money over the years from their service of correcting Harley rocker arm geometry), then that still means that when you increase valve lift via a performance cam, you have also changed the stem protrusion numbers which should theoretically retain correct geometry.

Here is basically how it works. If you were to draw one imaginary line through your pushrod and another through the rocker arm's ball socket to the center of the rocker shaft, when your cam is at one half of its lift, the line should form a 90 degree angle. Likewise, an imaginary line from the center of the rocker shaft to the pad of the arm should also form a 90 degree angle with the centerline of the valve stem at that same half lift point. That way at zero lift the line through your rocker arm should be the same amount below 90 degrees as it is above 90 degrees at full lift. But that means that if you increase the lift of the valve with no other changes, then the angle with the valve closed will remain the same , but the 90 degree relationship between pushrod and rocker will no longer be at 1/2 lift. To get back to the theoretically correct valve train geometry you would need to lengthen the valve by an amount equal to 1/2 the increase in lift. Or, you could get the same effect by sinking the valve that amount. And guess which is easier and more cost effective, sinking the valve or having a custom valve manufactured?

All of that is to say that with a performance cam, the theoretically correct stem protrusion increases at a rate of half the increase in valve lift. In practice this also has the added benefit on a Harley of providing the increased valve to valve clearance during overlap (commonly referred to as Top Dead Center lift) which is needed for those performance cams.

Now, with all that to digest, I'll pause briefly before continuing with the question of valve stem protrusion on a Knucklehead.  Stay tuned.

Aftermarket Knuckle Heads

This is not the sort of post that I enjoy writing.  I would much prefer to write a glowing report on a good product.  On the other hand, these things are not cheap, so I do believe it is reasonable to sound a warning so that buyers can make an informed purchase. 

Reproduction Knuckle heads from V-Twin Manufacturing.  The "issues" I will list range from minor annoyances to full blown problems.

First let's look at the annoying things.  The black paint on the heads is thin to the point of being translucent in many places, except of course in the areas that have runs.  As it turns out the runs are just as annoying as the see though if you opt to blast it off for a full refinish since the runs do a pretty good job of resisting removal with glass beads.

Speaking of paint, the heads were obviously painted before machining operations were performed, leaving large areas of bare cast iron.  Notably missing paint are the large spark plug "wells" and the tops of the rocker box supports.  Perhaps for this reason the machined surfaces were not deburred leaving sharp edges, many of them sharp enough to cut you quite easily when handing.  As it turns out, that can be VERY annoying.

The set I purchased was the version without rocker boxes and shafts, so they came without the upper rocker cover tins installed, but gaskets for them were included.  Upon inspection, they found a new home in a trash can, since they were so dried out and brittle that clearly  any attempt at installation would have been an exercise in futility.

The spring cups, or lower covers as they may also be called, have a nice Parkerized finish, and seem to be fairly good stampings (better than those from the same vendor a number of years earlier).  The brazed in oil return lines, however, have a copper plating which does not "take" the Parkerizing.  That may be a good thing in some ways, since the color may discourage some from trying to pass them off as originals.

On to what I consider a little beyond annoyances.  The valve spring are green.  So, what do you have against St. Patty's day, you might ask.  Well, green is not my favorite color, but this goes a little past interior decorating choices.  The valve springs are painted green.  In fact they are thickly painted green, possibly with a brush from the look.  Thick to the point of chipping off.  I can't say just what chunks of this green paint would do to an engine, or where it would ultimately wind up, but its certainly not something I would want to take a chance on.

The heads come with plumber style intake nipples installed.  In fact, they are installed with a "stock style" rivet to keep them from turning.  And when I say "stock style rivet" it is because the rivet is of the 1/4" diameter oversize normally reserved for a damaged hole on a used and abused head.  Too bad about that too.  Once the oversize rivets were removed, the nipples could be removed easily enough, meaning they were not installed very tight to begin with.  That added to the fact that the nipples had been installed with no type of sealer, leads one to conclude that the possibility of intake air leaks would be somewhere between likely and inevitable.

Next up is the alignment of the spring cups.  Keep in mind that these are held in place by being sandwiched between the valve guide and the head, so they need to be in the right position before the guide is installed all the way.  I keep a spare set of rocker boxes (knuckles) with shafts (no arms) among my special tools for just this purpose.  That way, I can insure that the spring cups align properly with the holes for the rocker shafts and that the other end of the cover fits into the hole in the rocker box.  In the case of this set of heads, not only did the rocker box end of the spring cups not line up with the knuckles, there was a good sized gap between the cup and ear on the head where the rocker shaft  passes through it.  Any attempts to correct this without driving the valve guides partially out would result in bending the lower spring cups, possibly with further damage.





Hard to tell form the poor photo, but this shows daylight between the spring cup and the rocker support ear on head



Going a little deeper yet, I found that the I.D. of the valve guides to be abnormally rough; not necessarily a good thing for longevity.  But, as it turns out, that would not be a problem after all, since the valve to guide clearance was too tight, meaning that by the time you honed the guides for more clearance, they would likely have a smooth interior.  Measuring the valve to guide clearance with a dial ball gauge showed .0022 to .003" on the intakes and .0034 to .0055 on the exhausts.  However, the fool proof final check for guides (at least in my book) is checking with a plug gauge (also known as a go/no go gauge).  This takes into account a valve guide bore that is not straight, something the ball gauge will not tell you.  In this case, a plug gauge .001" larger than the valve stem diameter would make it through one intake and and .002" larger on the other intake.  On the exhaust side a plug gauge .002 larger would go through one and .0025 larger on the other.  Stock clearances for a Knuckle are .004 to .006" clearance on both intake and exhaust.  I cannot imagine that these heads would have lived at these clearances.

The next item is only a problem if you plan to use these on a 61 inch motor.  The counter bore for the fire ring in the head is a slight interference fit on an OEM 61 inch Knuck cylinder.  That doesn't affect you if you have a 74 inch since there is no fire ring on the larger cylinder. 

The last item on my list does not really fit into either the annoyance or the problem column, and so possibly not worthy of mention, but I won't let that stop me.  Both exhaust valves had been treated to an approximate 45 degree cut on the combustion chamber side of the O.D.  I have seen the claim in print that this enhances flow.  It does not.  My guess is that this is an old wives tale started because someone saw a set of high performance ported heads with this modification and assumed it was for better flow.  In actuality, this extra angle is a last resort option for valve to valve clearance during overlap.  If one had a finished set of ported and flowed heads with less than the minimum required valve to valve at TDC, one might cut an angle on the margin of the exhaust valve only (because the resulting flow loss would less costly there).  Many years ago I tested this on my flow bench, and the result was so profoundly bad that it made an impression.  The margin thickness on a valve can have a large effect on its flow characteristics.

That pretty well wraps up this product review.  As I stated at the beginning, I wish I had better things to say about these heads, but there is this:  I think we can all be thankful that V-Twin put these heads into production.  The supply of repairable original Knuck castings is fast dwindling.  I believe they would be a better value if they were available as bare castings, but still, better something to work with than nothing at all.

One More Thing for Us Knuckleheads to Worry About

Or, to be more accurate, one more thing for those of us who are Knucklehead aficionados to worry about; but that wouldn't have fit in the title box.

One of my long time customers who goes by the name "Tom" (and I suspect it may be his real name) has had the same Knuckle for as long as anyone around here can remember. Over the years, I have had my hands on and into most of the parts of his engine, but never all at once. You see, Tom is just one of many Knucklehead owners who like to do as much of their own work as possible. Back when many of us started riding Knuckles, this was often out of necessity as much as choice, paychecks being what they were.

In any case, somewhere between the turn of the century and 2005, Tom had me order up a set of new reproduction Knucklehead tappet blocks for him. I really don't recall the specific occasion, whether it was a broken flange, or maybe it was Valentine's day and he wanted a special treat for the old gal. I ordered a set from Flathead Power.

At that point in time, Flathead Power was in no way connected with S&S - it was still "in transition" between its founder, Anders, operation in Sweden, and the acquisition by S&S. That transition period, and the unscrupulous characters involved have been gently exposed elsewhere, so I won't comment further.

Tom installed the FHP lifter blocks with a new set of lifters, and went on his way - which way consisted of putting lots of miles on his '42.

Fast forward through several years of hard riding, and Tom found himself in the need of a complete rebuild following a catastrophic parting of the ways of many of the engine parts which formerly made up his '42. The word grenade could be aptly applied.

As part of the resurrection, Tom opted to use another Flathead Power product which he had acquired, that being a set of FHP Knuck heads (also pre S&S vintage). Here again I was called into the act to provide oversize intake valves along with porting work. Knowing that the master plan called for a high lift cam (by Knuckle standards) I was very careful to get the stem protrusion on all four valves in just the right place to provide for a combination of the correct valve spring installed height and retainer to seal clearance while keeping the guide as long as possible to promote longevity. That is why what happened next left us scratching our heads.

Because of the special order 3-5/8" bore pistons which were chosen to match the 4-9/32" stroke flywheels and provide his targeted compression ratio, Tom mocked up the new engine parts with clay in the valve reliefs to be sure clearances were sufficient. Immediately a discrepancy became apparent. The rear intake valve to piston clearance was approximately .100" more than the front intake. Puzzling to be sure, but a problem...? If this had been a well used set of heads with untold numbers of past valve jobs, it would not have been too surprising, but that was not the case here. Suspecting a possible, albeit unlikely problem with the new Leineweber cam, Tom substituted his older, milder Leineweber cam. Same difference in clearance front to rear. Yet another check with a stock Knuckle cam gave the same results. Mr. Leineweber's name was thereby cleared of any crime in this case.

At this point I was starting to worry that I had gotten the stem protrusion off by .100" on one valve - not such a stretch given the human capacity for mistakes and the fact that my stem protrusion measuring device features a dial that measures .100" per revolution. I had Tom bring the heads back in so I could double check my work. To my relief and consternation, the problem was not in the stem protrusion.

My next thought was that perhaps there was a difference in the height of the machined surface that the valve guide bottoms out against from one head to the other. Some careful measurements which involved placing the head gasket surfaces on a table and dropping a steel rod through the guides proved this also was not the culprit.

I was nearly out of ideas, but there was one logical check still to make. I set up the lower end for my '46 with a degree wheel to perform what is commonly known as "degreeing a cam." Finally! By checking the opening and closing events with both a stock set of lifter blocks as well as with Tom's FHP blocks, it was obvious where the problem lay. The rear intake was opening 27 degrees later on the FHP block compared to both the stock block and to published specs for the cam (measured at .020" lift). And of course, checking the tappet lift at TDC, showed the difference between the valve to piston clearances which Tom had found with clay. The timing for both the exhausts and the front intake all matched those taken from stock lifter blocks. Now the only puzzling thing left to this story is that there was no large noticeable loss of performance when Tom installed these lifter blocks! I certainly would not have expected that.

The drawing below should help you visualize what the problem is, despite the sore lack of artistry. It is not to scale whatsoever, but shows the relationship between the tappet bore and the cam lobe centerline in a way that (I think) makes it understandable.

However, a little closer look at the figures gives a clue. Looking at the .053" lift timing, the difference between the FHP and the stock lifter block shrinks to 20 degrees on the opening side and 16 on the closing. That puts the timing on this rear intake, when used with this performance cam, pretty close to that of a stock Knuckle cam. In essence, for several years, Tom's '42 was running with a "hot" cam on the front cylinder, a hot cam for the rear exhaust, and a stock cam for the rear intake. My guess is that had he been running a stock cam, the performance loss would have been much more noticeable.

An important note. When this problem became evident, I contacted the gentleman at S&S who heads up the Flathead Power division and was pleased to find out that they had found the same issue and corrected it before they went into production, so you can rest assured that if you purchased FHP lifter blocks after S&S took over, they are machined correctly. I procured a new S&S block for Tom and he can attest that the valve to piston clearances now match front to rear.

One other note. I have no reason to think that ALL the pre S&S FHP rear lifter blocks suffer from this defect. It could very well have been just one run of the blocks made during that "transition" period. But that brings us back to the title of this post, doesn't it?

At Long Last ...

It certainly seems as though this has taken an awfully long time. More than once I have wondered if the rapture would arrive first ...but I finally have, in my possession, the production version of the oversize Knucklehead intake valves that have been in the works for well over a year. At a 2.060" diameter, these are a full 5/16" larger diameter than the stock valves they replace.

Bigger intake valves in Knucklehead motors has been a "speed trick" for nearly as long as Knuckleheads have been built. These valves are the proper diameter to "do it old school" by removing the existing seat insert and grinding the new seat directly on the head. By blending the material from the bowl into the I.D. where the old valve seat was, you will wind up with a "choke" of 90% which is just about right for good flow on a Knuckle head.

Back "in the day" automotive valves were typically shortened for use as oversize Knuckle intakes. While period correct, they have a few drawbacks. Shortening the valves require the use of a lash cap on the valve tip to prevent premature wear, which can throw off rocker geometry. Often automotive valves have a lot of "tulip" which requires the bottom of the already short Knuck guide to be further shortened. They also tend to be very heavy, which does nothing for performance. These 5/16 stem valves address all of those problems.

For some time now, savvy engine builders have been aware that a late model Iron Head Sportster valve was a good choice for an oversize intake valve for a Knuckle due to their similar length. Though the XL's 1.940" diameter valve was easy to come by, its thinner (5/16" vs 3/8") valve stem still left it a "not so easy" upgrade. No problem getting valves, but few engine builders are inclined to machine their own valve guides. That's why I took the initiative to have a production run of 5/16" bore valve guides manufactured for Lee's Speed Shop recently. Of course that still leaves an issue with the stock valve seat inserts. A 1.940 or a 2.0" intake valve does not fit onto the stock Knucklehead valve seat insert, and requires machining for an oversize valve seat insert.

Not so with the 2.060" valve. As mentioned above, once the old seat insert is removed, a seat for the new valve can be ground or cut directly on the cast iron of the head (just as Iron Head XLs have always been). This 60+ year old performance mod for Knuckleheads has never been so easy! They are now in stock, and ready to ship.

Now, chances are, if you are paying close attention you will have caught on to another logical use for these valves. If an Iron XL intake valve is a suitable upgrade for a Knuckle, then it follows that an oversize Knuckle intake valve may be a suitable upgrade for an Iron XL! That's right, a 2.060" intake valve may (I have not had a chance to try it out yet) be a good bet for a maximum effort Iron XL drag motor. In case anyone is interested in trying it, the overall length of the 2.060" valve is 3.675 as compared to the R series XL intake at 3.690.

If you are wondering why anyone would set out to produce and market performance parts for a motor that has been out of production for over 60 years, then you are in good company, for I often wonder that myself. Still, it does seem that I have a corner on the market ...at least for the moment.

Tire Change Video

Knucklehead Tire Change - How to Video

The first step is to make sure that you really need a new tire. Humor.

(finally gave up on trying to upload this, and put it on You Tube instead)

Cone Knuckle Part 2

When we left off in part one, we had looked at and discussed four of the five issues relating to running Knuckle heads of a Shovelhead Cone lower end. We talked about rocker arm ratios, lifter blocks, cams and pushrod tube angles. That leaves the toughest issue for last; oil return from the top end.

Perhaps a review of how the stock Knuckle oil return system works is in order. Starting on one end of the system, the Knuckle breather gear has a hole that is not found in any later breathers. This hole is timed to provide crankcase vacuum to a passage (also unique to Knuckle cases) which connects to the Knuckle lifter blocks. That is what the fifth hole in a Knuckle lifter block gasket is for. In this way, crankcase vacuum is applied to the pushrod tubes and the aluminum housings which give the Knucklehead its name.

Note that this passage in the Knuckle lifter block goes to the top of the lifter, with no drain hole into the gearcase such as on a Pan or Shovel. In fact, if you have replaced your Knuckle lifters with any aftermarket lifter (with the exception of the ones from S&S, which are made correctly) there is a good chance that you have a hole drilled through the lifter which will bleed off the vacuum that is needed to evacuate the heads. This hole is not supposed to be there, and could very well be the root cause of your smoking and oil consumption on your stock Knuck.

The reason that this vacuum in the pushrod tubes and Knuckle housing is critical, is that the 'spring cups" depend on that vacuum to "suck" the oil, through the 3/16" O.D. Steel lines uphill into the housings. If you have ever had a Knuckle head apart, you are no doubt familiar with the seals that are between the aluminum housing and the tin covers. These not only seal the assembly from leaking to the outside, but they also separate the two sides of the rocker arm to keep vacuum on one side only.

Compare this to the oil return on a Shovel, Pan, or Evo. All of them return the oil from the heads via a passage through the cylinder directly to the crankcase. They basically depend on gravity aided by crankcase vacuum when the pistons are on the upstroke (or crankcase vacuum aided by gravity, if you prefer).

So, what are you to do, if you have a set of Knuckle heads, and a Cone lower end you are dying to put them on? It looks as though there are a few possibilities. One which I have performed is to rework the steel lines coming out of the spring cups so that they don't run uphill as much as in the stock application. From there grafting on soft lines and routing them externally down to the crankcase roughly adjacent to the timing plug converts the drainage from the spring cups into the gravity/crankcase vacuum style like Pans and Shovels. The oil from the aluminum "knuckle" returns down the pushrod tubes.

A second method comes directly from a comment received on what I posted in part one of "Cone Knuckle." Richard writes that he has been running a Knuckle-Cone motor since 1986. He uses Shovel lifter blocks with a brake tube brazed to the intake pushrod tube and plumbs this into the fitting on the crankcase that returns oil from the primary in stock configuration. This puts timed vacuum to the heads much like the stock system and so avoids any mods to the spring cups, etc. I think this is an absolutely brilliant idea, and though I had considered the idea of using the primary return, my thought was to go directly from the spring cups to the primary return. Richard's system is much simpler and cleaner! Thank you Richard!

But, now there is one more viable option. It turns out that I have a reader who works for S&S Cycle. After reading part one, he informed me, and got permission for me to break the news, that S&S is introducing a complete Cone Knuckle engine this summer. The S&S version will be called the Kone Knuckle, and will feature appropriate passages and machining to use Knuckle lifter blocks, and hence a stock style oil return system. Plans are to also make the cases available for those of you who have a bunch of Knuck parts stashed away for just such a project. This will make the whole process relatively painless.

Keep in mind though, that excess clearance between the Knuckle rocker arms and shafts can be a source of excessive oil in the top end, enough that even an otherwise correctly working return system will not keep up with. A rebuild of those parts may still be in order.

So, just one last thought. Whoever said that Cone Knuckleheads couldn't/shouldn't be built ....well, aren't you glad that neither Richard or S&S listened to them.

Cone Knuckle Part 1

Knuckle heads on a cone style Shovel bottom end. Most have seen one, so there is not much debate as to whether it can be done, but there is much disagreement as to whether it should ever be done. Those who say it shouldn't be done generally fall into one of two camps. The purist, who believes that it is sacrilege to desecrate a set of Knuckle heads by running them on anything but a set of Knuckle cases, and the quasi engineers who see nothing but disaster when a part does not work exactly as the designer envisioned.

On the other side of the issue are two likewise divergent camps. On the one hand you have the less-than-a-shade-tree mechanic who will throw together nearly any combination of parts that will physically fit together and leave the "making it work" for the next owner. On the other is the wrench who does his best to weigh the issues and address them in such a way that the finished product is functional as well as unique.

Having raced a Knuckle top/ Cone bottom drag bike for a number of years, I can attest to the fact that those who say that there are too many cam/rocker arm/geometry problems for a motor like that to run well are wrong. Two AMRA National Championships would suggest that they are very wrong. In fact, performance is easily addressed by standard methods. Making the motor streetable in regards to the oiling system is a much more difficult proposition.

There are a number of things to consider before building a Cone Knuckle. In no particular order, they are:

1. Different rocker arm ratio. Knuckle rockers are 1:1. Shovel rockers are 1.43:1. That means that a cam that lifts the valve .450 in a Shovel will lift it .315 in a Knuck. (.450 divided by 1.43)

2. Knuckle lifter blocks don't bolt onto Cone cases. Some of the bolt holes are close, but no cigar, and there is no oil port in the case to match the one in the Knuckle lifter blocks.

3. A Knuckle cam will not fit in a Cone cam cover (at least without major modifications).

4. The angle of the pushrod tubes is different between a Knuckle and a Shovel or Pan making it difficult to seal the bottom of the pushrod tube to a Shovel/Pan lifter block.

5. The oil return system from the heads is completely different between a Knuckle and a Shovel, Pan, or Evo.

Looking at the rocker arm ratios first, there are a number of ways to approach this. One is to have a cam custom ground with Knuckle profile lobes for use with Shovel lifter blocks. This solves any and all valve timing and lift discrepancies, but will be expensive and time consuming.

The second method would be to select a high lift Shovelhead cam. Stock and mild performance Knuckle cams are from .350 to .370" lift. Taking into account the Shovelhead rocker ratio of 1.43:1, you can see that a Shovel cam that is advertised as .500 to .530 lift will open the valve .350 to .370 in a Knuck. If you consider the S&S 514 Shovelhead cam, its .514 lift translates to .359' lift with the Knuck's 1:1 rocker ratio. Compare its other specs with two Andrews Knuckle cams; the N is considered a stock replacement and the S a performance cam.

S&S 514 - IO 23/IC 43 (246 duration) and EO 43/EC 23 (246 duration)
Andrews N - IO 13/IC 41 (234 duration) and EO 44/EC 16 (240 duration)
Andrews S - IO 27/IC 55 (262 duration) and EO 55/EC 27 (262 duration)

As you can see, the S&S 514 falls nicely between the specs of a stock and a performance Knuck cam. Seems like a no brainer to me that this (or a similar cam grind) would be a viable choice.

Using Knuckle lifter blocks on a Cone lower end, while possible, would require quite a bit of welding and machining as well as a custom ground cam (or even more machining to fit a Knuck cam to a Cone cam cover). Not really practical unless you are a machinist by trade and just want to show off.

If you have by now settled on using Shovel lifter blocks, the next issue is the angle at which the pushrod tube meets the block. Now back when I was racing, this was a non issue, since I ran no oil to the top end, and no pushrod tubes. This lack of pushrod tubes did, however leave me with a couple memories that will give a clue as to the solution to the problem of sealing them up. On my Cone Knuckle drag bike the pushrods appeared to be perfectly in line with the lifters. I mean straight as an arrow (at least to the naked eye).This was especially noticeable in comparison to the Shovel dragbikes which were its contemporarys. The pushrods on a Shovel were at a fairly drastic angle coming off the top of the lifters. One might never notice that if they have never looked at a Shovel motor with pushrods installed, but no covers. The reason is because the Knuckle has four different lengths of rocker arm so each one aligns with its corresponding lifter. Not so on a Shovel (or Pan, or Evo, or even Twin Cam). The misalignment on these "modern" motors is compensated for by the ball and socket arrangement on each end of the pushrod.

If you look at a Shovel lifter block, you will notice that the "seating surface" for the cork seal is NOT perpendicular to the bore for the lifter. Well, one of them is probably perpendicular, but the other three are canted to line the tube up with the rocker/box. I have not had a chance to try this, but I suspect that aligning the pushrod tubes on a Cone Knuck may be as simple as remachining the seating surface in the blocks so that they are perpendicular to the lifter bore.

Finally, we come to the oil return system from the heads. Oil return from the heads is the biggest obstacle in making the Cone Knuckle streetable. Since it is fairly involved, it will have to wait for another post - coming soon in part 2.

Prepping Knuck Heads for Big Valves

I intended to post something entirely different, but at the moment, Blogger is not letting me cut and paste, so rather than rewriting something I have saved in another program, I will do something fresh.

Some time ago I started a series on installing bigger intake valves in a Knuckle head. This will be the second part of that series. When I left off, I had finished installing new exhaust seats. The seats from Rowe machine nicely (something that cannot be said for the 60+ year work hardened original seats. This also allows you to set the stem protrusion of the new valves, avoiding mis-matched depths.

If the intake nipples were removed for replacement (and in most cases they should be), now is the time to re-install them. I like to use JB Weld on the threads of the nipples, along with a stock type rivet. The epoxy is particularly important when porting, since some of the inner threads inevitably will be ground away. Here's a tip: from here on out, keep a set of used intake nuts screwed on to the nipples to protect those nice new threads.

Now that the new seats and intake nipples are installed, it is time for porting work, if you plan to do so. Doing this before new guides are installed allows you to do the best possible job. Disregarding the area just below the seat and the short side radius, the remainder of both ports should be shaped and finished as per the final product. I like a 50 grit on the intake, and a polished exhaust. However if the exhaust port is to get a thermal barrier coating, there is no point in spending extra time doing a fine polish.

With the majority of the porting work done, it is time to paint the heads. This needs to happen before guide installation, since the spring cups are held in by the valve guides. For a stock look, I use a semi-gloss black, and bake it on at about 200 degrees. Many of the hi temp paints available need this heat cycle to enable them to stand up to standard parts wash fluid. Nothing worse than keeping the new paint looking good through the whole process, only to have it get sticky and wipe off during the final wash prior to final assembly.

While the paint is drying on the heads, it is a good time to make sure the spring cups are ready to go. Often you will find a small crack just below the return tube. Be sure to weld these before going any further. Next, all of the gasket surfaces on the cups should be attended to. I use several purpose built forms along with a small hammer to return them to their original shapes. Don't forget to re-flatten the bottom portion that the guide will seal against. Once the cups are all in shape, they can be Parkerized if you are going for a stock look.

Guide installation, cutting the seats, and final porting work will be covered in the next installment.

New, Stronger Cylinders for Your Knuck

I received a set of S&S/Flathead Power 74 inch Knuckle cylinders into the shop last week, and thought there may be an interest in some comparison shots. The black powder-coating on the S&S cylinder on the left is a little too glossy for my taste, but will doubtless hold up well.

A nice touch is the cast in emblem which suggests (but does not copy) the original's MC (Milwaukee Casting) badge

Cast in number plate also pay tribute to the original, though on the Flathead Power cylinder the large "F" corresponds to Front cylinder. Note the crack in the spigot on the stock cylinder.

Even the pistons that come with the cylinders have a dome that is reminiscent of their vintage counterpart. On left is the new S&S cast 74" piston, on the right an English made replacement piston from back in the day.

Currently the S&S suggested retail for a set of these cylinders is $980 with pistons, or $850 for cylinders alone. Not cheap by any means, but certainly a worthwhile investment if you plan serious performance modifications.

Knuckle Seat Install

As I mentioned previously, one of the items I would like to cover on my blog this year is installation of big intake valves in a set of Knuckle heads. If you are dealing with stock heads, it is quite likely that you will find good reason to give the exhaust seat inserts attention also. Sometimes the stock exhaust seat is simply sunk too deeply from multiple valve jobs. Often the seat will be pitted so badly as to be unusable. But even if those two items are not an issue, you may want to consider replacing them because they have become extremely "work hardened" from 60+ years of use. While this extreme hardness is not an issue while in service, it makes the seats very hard to work with. Modern seat cutting equipment will invariably chatter, and stone type seat grinders will require lots of time and constant dressing if there are any misalignment issues to be corrected.

So, as part one of this series on installing bigger intake valves in a Knuckle, we will look at replacing the exhaust seats, though if you are planning to use stock size intakes, the same procedure can be used for those seats.

ABOVE: Removing the old seat inserts is pretty simple on a Knuck. With the guide removed, a slim punch will fit though the guide hole and can be angled to catch the back side of the insert. A few taps and your heads should now look like this. (both intake and exhaust seats removed)

ABOVE: Once the seat recess has been cleaned, carefully measure the bore in multiple places to determine how much press fit the new insert will have. You want a minimum of .004" interference, though I prefer .005"-.006".

ABOVE: A home made tool will work for seat installation. This one is made using an old valve with a collar welded just below the face to keep the new insert centered, and a piece of scrap for a handle. Shown here with new insert not seated on collar.

ABOVE: Tool with new seat insert seated on collar. A "dummy" guide will have to be installed temporarily in head to keep everything aligned so that the seat goes in straight. You can always use your old guide (you didn't throw it away did you?), sanded down a bit so it is only about a half thousandth press in the head.

ABOVE: Prior to actual installation, put the head in your oven at 500 degrees (that is the head in your oven, not your head in the oven) and the seat insert in your freezer. If the head is clean, it should not stink up the kitchen, but if it isn't and your wife catches you, you may want to go back to putting your head in the oven. Unlike with an aluminum head, even with a heated head and frozen guide, you will have to use moderate blows with a hammer to install the seats.

ABOVE: Here is the head with new exhaust seat insert installed. Now you can remove the dummy guide and proceed with the valve job, which will be covered in a future post.

One last thing, if you are having trouble finding suitable valve seat inserts, you can follow the link at the bottom of the left hand column of this blog to my eBay store. I will generally have what you need in stock.

Lightning Cam Pics?

In my last post I featured a Leineweber # 5 Knuckle cam. This elicited a response from a reader, Jim Franco, saying that he has a cam that recently came out of a Knucklehead hillclimber with lobes that looked quite similar. He was kind enough to send me some pictures of the cam, which I have posted here. The holes in the drive gear immediately bring to mind the legendary Knucklehead "Lightning" cam. Contrary to what seems to be popular opinion, I do not believe that the Lightning cam was the only factory cam to have these holes.

As can be seen in the above picture, the lobes do have the radical shape similar to the Leineweber #5. I have heard the Lightning cam referred to as the "banana lobe cam." I am not sure if this was a common slang term for it or not, but one can certainly see where the name came from.

I ran a factory Knuckle cam which had holes like this in my street bike for a few seasons. The lobes did not visually appear any different from any other stock Knuck cam, and it did not perform noticeably better. I also do not remember a keyway, like the one seen in the above picture.

Barring someone with more knowledge of the elusive Knucklehead Lightning speaking up, I am very inclined to conclude that Jim has himself a genuine one here!

That's Not a Cam; This is a Cam!

Thought you might enjoy seeing a somewhat rare cam. This is a Leineweber #5 Knucklehead cam. It is .540" lift with the Knuckle's 1:1 rocker arm ratio. The description for this cam is as follows: Radical racing cam. Designed for fuel, alcohol, or gasoline. Major engine modifications needed to fit this cam. Expert engine builder a must.

Is that cool or what? The lobes at 8 and 12 o'clock are the intakes, and the milder exhaust lobe is the one at 3 o'clock. Jim Leineweber only lists his Knuck cam specs at .020" lift. For this cam the duration is 318 degrees on the intake and 298 degrees for the exhaust. The picture doesn't do it justice as to just how radical this cam looks!

Twenty + years ago a local drag racer from the '60s and 70's, the late Doug Gall, showed me a cam which Jim Leineweber had ground for him. He told me it was one of Jim's first. It was a stock cam that had been welded up and reground. As I recall, it had much this same shape to the lobes.

The best part is you can still get one of these for your own nitro burning Knucklehead at leinewebercams.com (or contact me).

Spark Plugs 101

Here's one that is frankly on the verge of being a little embarrassing. When I bought my first Knucklehead nearly 30 years ago it had a set of Champion D14 plugs in it. For about the next 20+ years, whenever I replaced plug on any of my Knuckles I would simply go to my local Napa and buy another set of D14s.

Then somewhere along the line, I became aware that the Champion D16 was the correct cross reference for the Knuckle. So, after all those years I was faced with no longer being able to remember which number was right and which was the one I had used for so many years. Have I mentioned lately that its no picnic getting old?

So for the last couple of years, every time I have been asked for a spark plug for a Knuckle, I have not been able to remember which was correct; D14 or D16. That all changed the other day when I was researching something else. I happened to run across this in Palmer's "How to Restore Your Harley Davidson":

• "Of the original Harley-Davidson 18mm spark plugs, the No. 3 is most often used. For a motor used mostly on the highways at fast speeds the No. 4 plug is more desirable."

And a few lines later:

• "A cheap alternative to the No. 3 and No. 4 18mm plugs are Champion D-14 and D-16 spark plugs, respectively."

Kind of reminds me of the old line "I used to think had made a mistake once, but I found out I was wrong." So, as it turns out either the D14 or D16 is an acceptable plug in Knuck, depending on the use.

I do think I have uncovered a small mistake in the Palmer book though. All the catalogs I have show the D16 as the replacement for the No. 3 plug. This is backed up by a Champion spark plug catalog that I have. The D14 is colder than the D16. The confusion no doubt stems from the fact that in Champion's heat range numbering the lower the number the colder the plug, and Harley did it the opposite way. That's a very minor discrepancy given the huge scope of his book. I have trouble not transposing stuff in just one short blog entry! The important thing is that there are interchanges for both the No. 3 and the No. 4 Harley plug.

With that in mind, here is a page from the Knucklehead service manual.

This drawing illustrates spark plug heat ranges pretty well. A spark plug's heat range refers to how well the plug dissipates heat. A colder plug dissipates the heat quicker, and so runs cooler. Therefore, a hotter plug will help keep deposits from fouling a plug if used in an over rich or oil consuming engine. On the other side of the coin, a colder plug will be less prone to overheating and causing pre-ignition in a high compression engine or one that is run hard (Both high compression ratios and prolonged high speed will generate more heat).

Interestingly, the factory manual states that:

• " In some cases best results may be found using a colder plug in one cylinder than the other. In this case it is usually the front cylinder that takes the colder plug as this cylinder is not as likely to foul a plug at low speed."

I've never tried it, but it seems logical enough. So next time you inspect your plugs and find they are not both the came color, it may be worth your while to try a different heat range! Now you know.