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."