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.
8 comments:
Interesting Lee! What advantages (if any) are there with the "square motor"?
And you mentioned one option of machining custom pistons..If a guy were to make them, what material would be best "Fortal"?
Opinions seem to vary on the actual or perceived advantage of a square motor. I think the real advantage is that a large bore gives room for larger valves, and thus more performance potential. A short stroke also keeps piston speed down for higher rpms (and rpms are the key to high horsepower). I believe most of the rest of it is more legend than fact (the mystique I mentioned).
When I spoke of custom pistons, I meant going to a piston manufacturer with specifications and having them build the piston. Many companies such as J&E are happy to do it, but they get a premium price for it, and often have a minimum order of 4 pistons. Despite the scenes from "The World's Fastest Indian" I don't feel qualified to start designing or making my own pistons.
That movie does make it tempting to give it a try though.
I hope no one was confused by my mistake. Under 68 inch racer I had listed "Cylinder height .200"+"
It should have, and now does say "Fire ring height .200"+" Sorry for any confusion.
Hi Lee, just found this blog, I was reading about the paper engines and the 68 inch racer, I have built few of these motors over the years and think you have the displacements wrong. The 68 incher is a small stroker 3.9687 (74) wheels installed in the 61 inch cases using the 61 inch top end, and the square motor that your talking about is a 65 incher 3.5 (61) wheels and 3.4375 (74) top end. I'm doing a 65 inch square motor for my EL in a VL frame bob job.
Hi Mark, thanks for stopping by, and commenting. In this particular case, we may both be right. My thought (possibly not articulated very well) was to use the 61" stroke with 74" bore of +.060. With a couple thousandths extra clearance for those pistons it comes out to a 3.5" bore x 3.5" stroke, or 67.4 cubic inches.
Your engine with 3 5/16' bore x 3 31/32" stroke comes out to 68.4 cubic inches.
It is very possible that you are talking about the correct combo for the term "68 inch racer". I had only heard the term from one person. He remembered that one of his Knuckle "gurus" from way back when talked of building them, but he did not remember the engine details. I was simply "winging it" with my version.
If you are speaking from knowledge of what was a commonly done modification in the past, it would be great if you would share any other details you might have. I'd like to think that we may be able to pass such info on to posterity by getting it out her on the Internet.
To use the 74" stroke in a 61" did you use stock pistons along with stroker plates and cutting the flywheel diameter down, or did you just shorten the piston skirts?
Now you've gone and gotten me intrigued all over again!
Lee, I don't if you'll remember me but we've talked knuckles many years ago, I own Early Iron Motorcycles formally of Hudson WI. now from De Soto WI. My first 68 incher was influenced by a guy out of Chicago who went by the name of Chicago Smitty, I built it with lightened 74 wheels using stock pistons that I shortened the skirts on and using stroker plates that I made so I could get as much squeeze in the chamber as I thought would be safe. I made up a set of custom valve guides to use cut down R valves, cleaned up the ports and run an SU carb. I ran a K grind cam and a pan dual point breaker, it ran real good I had no trouble out running my buddies shovels. I had it geared up as I was doing a lot of tripping back then, 24 motor sprocket and a 24 trans. The next one I built was pretty much the same except I had custom pistons made by Ross that brought my cr. up to 10.1, I built a custom single fire breaker using a Dyna S and I dual plugged the heads and ran a Primo 1.5 belt belt drive, still went with the 24T trans sprocket. This one went vary good, could out run my buddy Joe's hot rodded 1985 Softail EVO that he spent a lot of money on built by Total Performance in Hudson, needless to say he wasn't vary happy. I also could run with my buddy Keven's '99 Sportster Sport with my wife riding with me, he to was surprised. I also built a 65 incher like the one your talking about but with a lot less compression and was not happy at all with it, but then I just put it together with not much thought. This time I'm going much more aggressive with it, I may use some of your ideas on it. Keep up the good work.
Mark.
Hi Mark,this is some great info. Would it be ok with you if I made a fresh blog post out of our discussion in the comments section? Only new readers are likely to ever see new comments on an old post, and I think many would find your input very interesting! It would also give your shop a plug.
Lee, That is fine with me.
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