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.