68" Racer Revisited

A few days ago I received a comment from Mark, of Early Iron Motorcycles in De Soto WI, on a previous post called Paper Engines. In that post I had proposed an engine combination using 61" Knuckle flywheels, UL rods, and overbored 74" cylinders. I called that engine a 68 inch racer. The term 68" Racer was one that I had heard from another old timer (i.e. my age) He remembered that he had a Knuckle "guru" from back in the day who talked of building them, but he did not remember the engine details. Without really thinking about it, I applied the name to my version.

Mark was good enough to give me a little insight into what the term 68" Racer originally meant. Mark tells me that his first 68 incher was influenced by a guy who went by the name of Chicago Smitty. Mark built it with lightened 74 wheels using stock 61" pistons with shortened skirts and using stroker plates made to get as much squeeze in the chamber as he deemed safe. He made up a set of custom guides to use cut down R valves (a slightly longer version of the 1.94 Iron XL intake), cleaned up the ports and used an SU carb with an Andrews K grind cam and a Panhead dual point breaker. According to Mark, it ran so strong he had no trouble outrunning Shovelheads.

Later Mark built another Knuckle, similar to the first, but with custom Ross pistons for 10:1 compression, dual plugs in the Knuck heads and a custom breaker with Dyna S single fire ignition. This one ran so good it was faster than a high buck modified Evo. (I know what you are thinking - just spending a lot to hop up a bike doesn't make it fast - but the guy who modified the Evo in question is the same guy that ported the Shovel heads for my friend Bobby's motor which set multiple national records)

Right about now, I would bet we have a few Knucklehead fanatics out there who are getting mighty excited about building something similar. Thanks, Mark, for sharing that info with us.

I think we can be pretty sure that Mark's version of the 68" Racer is the "period correct version." It also has the rather large advantage of using the nice thick, pretty much indestructible 61" Knuckle cylinders. The only drawback that I can see, is the small bore size would tend to limit the intake valve size quite a bit. I'll have to take a closer look at that as I continue with my big valve conversion project.

68 Inch Racer, part 3

In part 1 and part 2 of 68 Inch Racer, I outlined the physical dimensions and bottom end parts for this "dream" motor. In this installment I would like to wrap it up with the top end and valve train.

As to the heads, if I had a set of dual carb Knuck heads laying around (which I do) I would probably use them. After all, if I am going to label this a "racer" then why would I be worried about the leg clearance issue raised by two carbs sticking out the left side of the bike. Intake valves; 2.060". I had a set custom made by Ferrea Racing for a motor I did last winter. Beautiful valves, and no need for a lash cap, but I was disappointed with their limited selection of tulip choices for a 3/8" stem diameter. The only one they offer had more tulip than I wanted, which necessitated shorting the bottom of the guides slightly. It was also quite a bit heavier than I would have liked. Perhaps a 5/16" stem next time with a custom made valve guide.

It goes without saying that the piston domes, exhaust ports and combustion chambers would get the ceramic treatment like this.

Working off a couple of Knuck heads that I have oiled out in the past, it looks as though a head volume of 128 cc is a realistic starting point. Now, if we bore the Knuck heads out for the Pan cylinder's fire ring, we actually increase the head volume by about 5.71 cc's. But of course the fire ring itself will take up most of that increase. The total head volume when bored for the fire ring, but taking into account the amount displaced by the fire ring, calculates to 128.38 cc.

But here is where it gets interesting. The 74" Pan cylinder is shorter than the 74" Knuck by .200" when measured gasket surface to gasket surface (5.530" vs. 5.330). But if you add the height of the Pan fire ring they are the same length. That means that the piston will come to the same point in relation to the gasket surface on the Knuck cylinder as it does to the top of the fire ring on the Pan. But in effect you are lowering the head .200" down by using the Pan cylinder (think of it like shaving the head .200") Now we are talking some compression! When I plug all this into my Engine Analyzer program with a .020 thick copper head gasket, it comes out to 12.26:1 compression ratio. And after double checking all my figures a couple times, it still comes out to that!

Cool. Now we are into race gas territory (68 inch Racer - remember). That kind of compression will allow us to run a fairly radical cam as well. Any time you talk about high performance on a Knuckle engine, in my book there is really only one brand of cam to consider, and that is Leineweber. I am thinking his #3 Knuckle cam. It has .420 lift and 312 degrees of duration, but keep in mind this duration is measured at .020" rather than the customary (for Harleys) .053". That is still probably near 275 degrees duration if converted to .053".

Keeping with the "racer" theme, everything seems to point toward an engine that will not be easy to start. So we might as well add a magneto. I'm envisioning a bike that does not even have a kick starter.

Since this "paper engine" is getting a set of real life vintage dual carb Knuck heads, I would also use a set of real life 40mm Dell 'Orto pumper carbs that I have left over from my first drag bike. They were a little small for the 114" Knuckle, but I suspect they would work well in this application. Since the heads have a 4 bolt Linkert pattern, I would weld flanges onto the Dell 'Ortos to match. Jetting them to run on E85 (85% ethanol) would be really tempting. It has enough octane for this kind of compression ratio, plus the government helps pay for it (with my money). Good luck on getting them to kick in for your $15 a gallon racing gas.

One last thing. If the motor was to be used on the drag strip, then I would leave the valve train open, with no oil going to the top end. This is not a huge advantage, it just simplifies the oiling system and eliminates any clearance issues between spring and cover. But that's the way we used to do it, and it looks pretty cool. For any other application I'd enclose the valve train. Either way I'd use a pre '40 oil pump for the thinner gears and less oil volume in the motor.

That pretty well wraps it up. Did I miss anything? And.... an even better question: will it ever get built?

68 Inch Racer, part 2

If you read my post entitled Paper Engines, you will recall that I solved the problem (at least on paper) of building a high compression 61" Knuckle lower end with a 74" top end.

There still are a few things to be considered in the bottom end. For one thing, there is the subject of flywheel weight. That will be partly determined by the end use of this motor. Whether it is for drag racing, "street" racing, Bonneville, flat track, or some sort of road racing. For most of these scenarios, I would lighten the flywheels (the one exception being drag racing with a slick). How much to lighten them is the question. The minimum would be to cut down the heavy left side flywheel to the thickness of the right side wheel.

stock 61" flywheels on left , lightened S&S on right

Bike weight is also a factor here. Flywheels "store" energy, so the heavier the bike, the harder it will be to get under way from a dead stop with extremely light wheels. Still, the gain in the rate of acceleration is too good to pass up in most applications.

Since the UL rods are an aftermarket imported item of somewhat unknown quality, I would at the very least have them shot peened. Possibly polished and shot peened. The purpose of polishing is to remove any possible surface imperfections that could lead to a crack. However, polishing also removes the surface left from the forging process which increases the strength of the rod. Shot peening puts that harder surface back on (so to speak). So, just shot peen, or polish and shot peen, but do not polish only!

Of coarse the rod change alone would dictate that the flywheels be balanced, but it becomes even more critical when lightened.

Since the point of this engine is to turn some RPMs, the crankpin and mainshafts are probably not a place to skimp. S&S makes all of the early shafts, including the stepped crankpin, and are probably the highest quality available. Yes the rod set will come with a new crankpin of unknown origin, but that would be better set aside to go in a restoration that will not see such severe service.

This is to be a "Racer" and not a restoration, so why not take advantage of the built in extra strength of Panhead cylinders. Even bored to +.060 (which makes it a true square engine 3.5" bore x 3.5" stroke) the Pan cylinders are undoubtedly stronger than a Knuck cylinder. Sure, there are reproduction Knuck cylinders that are stronger than stock, but they aren't cheap. I would spend the money elsewhere. Besides, there are plenty of Knuck heads out there that have been converted to the Pan headbolt pattern already, so why not use them.

For pistons, I would start with a cheap Pan/Shovel cast piston which is called a 10:1. The dome volume on them is about 52cc which will get some compression ratio (I'll discuss that more when I get to the heads). Once the domes are ceramic coated and the skirts Teflon coated they should work very well.

More to come.

Paper Engines

It is probably more common in the automotive field to dream up engine combinations on paper that will likely never be built. Sometimes though, something triggers that urge in my tiny little brain too. Of course, working in the Harley industry, that is usually the type of engine that figures into my daydreaming.

With the recent advent of relatively reasonably priced reproduction UL connecting rods becoming available, those tiny little gears in my head started turning. For me, it is not the designing of a motor with numerous custom (read: expensive) built components that gives me satisfaction. The combination also has to make some sense from a monetary standpoint to make it "cool" in my mind. That obviously stems from some long lost and unrecorded Scotch ancestry. In other words, yes, I am cheap!

With that in mind, I present this for your approval (or disapproval):

68 Inch Racer

In the past I have touched on the dilemma of 61" Knuckles or Panheads running cylinders that have been bored to the 74" bore size. Sure you can use 74" pistons with no clearance problems, but with the stroke of the 61" flywheels, the piston will not come to the top of the cylinder, dramatically lowering the compression ratio. This is not really a problem for your typical restoration or Sunday afternoon cruise bike, but to the true gear head, it is an unwelcome compromise.

Besides, what about the legendary "square" motor? (a "square" motor is one with the same bore as stroke) While there may be more hype to it than substance, there still is a certain mystique involved. And with the 3 1/2" stroke of the 61 and 3 7/16" bore of the 74 you are almost there!

As I mentioned above, the availability of UL rods brought this into my mind. The easy way to get that piston to come to the top of the cylinder when using a 74" piston on a 61" flywheel is either a custom made piston with the wrist pin location moved, or a custom made longer rod. Either of those can easily be accomplished if you are willing to spend a lot of money. But like I said, that kind of spoils it for me. But the UL rods are longer than OHV rods. Hmmm.

Here's what it looks like to me:

Stock 61" Knuckle

• Case deck height (center of main bearing race to cylinder gasket surface) 5.380" +
• Cylinder height (gasket surface to gasket surface) 5.405" +
• Fire ring height .120" +
• Base gasket .020"
• = total height of 10.925"
• 1/2 of the stroke (distance from center of mainshaft to center of crankpin at TDC) 1.750" +
• Rod length (center to center) 7.46875" +
• Piston compression height (piston deck to center of wrist pin) 1.625"
• = total height of 10.84375

Subtracting 10.84375 from 10.925 we see that the piston should be about .081" below the top of the fire ring.

Now here is the same calculations on my proposed motor:

68" Racer

• Case deck height 5.380" +
• Cylinder height (74" Panhead cylinder) 5.330" +
• Fire ring height .200" +
• Base gasket .020"
• = total height of 10.930"
• 1/2 of the stroke 1.750" +
• Rod length ( UL rods) 7.90625 +
• Piston compression height (generic 74" piston) 1.415"
• = total height of 11.07125"

Here we see that the piston would come out of the top of the fire ring by about .141". But if we add a .125" "stroker plate" (and another .020" gasket to seal the other side of it) it will put the piston deck about .004" below the top of the fire ring. Just right!

That covers the main configuration, but leaves a lot of details to be discussed. I'll continue this "paper engine" in my next installment.