Friday, March 12, 2010

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.

5 comments:

St.Lee said...

I had a comment via email which I thought I should share parts of.

Matthew wrote:
"....I have had the pleasure and pain of being a knuckle jock since the seventies.... Your version, your original version is the correct one. A square motor it was sometimes called. The small bore version, well I am sure it would run, but the 3 1/2" bore 3 1/2" stroke motor is tried and true for 60 years.... Square motors are where it's at."

So, I guess there is more than one way to skin a cat. And when that cat is a Knucklehead, I guess you can't go too far wrong no matter how you go about it.

Doing cylinder head porting for a living, I have long noticed that Knuckles far out perform what one would expect from just looking at their flow. Anyone have any thoughts on that?

It is unfortunate that most of the locally legendary Knuckles are long gone in this age of Dynamometers. Anecdotes are fun to reflect on, but don't offer a lot of solid proof of actual performance levels.

Is the world ready for a Dyno shoot out with Knuckleheads exclusively?

CycleDelicCycles said...

Hi Lee,
I havent seen any dual-plugged knuckle heads, although I did have a pair done for a 93" shovel I had, and the results were far beyond my expectations... I had noticed prior to dual-plugging them that the far side of the piston always had the carbon buildup as opposed to the "clean" plug side. Does this also hold true for knuckleheads? I should have my FHP stroker done in a week or so (thanks for the pushrod tubes!)and am just trying to familiarize myself with knuckle-isms. Also, on the shovel, I used dual single-fire coils for the dual-plugged heads (which worked great)... would this be the setup to use on the dual plugged knucks or....? Thanks in advance for the insight. John in FL.

St.Lee said...

Hi John, since Knuckles and Shovels share a very similar combustion chamber, you should get similar results. I have never had anyone ask me to dual plug a street Knuckle, partly because it detracts from the vintage look, and partly because of the logistics. If you look at Knuckle pushrod tubes, they have less room between them at the top than a Shovel, so there is less room to get a plug wrench in. There is also an issue with the angle of the plug and clearance to the rocker box.

I ran dual plugs on my Knuckle drag bikes (so did Pete Hill) but without pushrod covers there was more room to work with. I suppose maybe it could be done for street use, but it would take some careful measuring and machining.

Juhana said...

Hello from Finland.
I'm really interested to build 3 1/2 x 3 1/2 engine out of early Panhead. Basically all your calculations and thoughts will work with Panhead similar way as with Knucklehead (?) I have 1952 cases, few sets of heads (one pair with huge valves, " seat savers" I guess, 61" flywheels etc...
I'm going to build it for street use in light bike. What kinda suggestions you have for carburator, cam, exhaust and ignition? And for compression on pump gas. (Our best quality is 98 octane with only 5% of ethanol). Also is it worth tryin to use dual carbs?
Br, Juhana H

St. Lee said...

Hi Juhana
A 3-1/2" bore, 3-1/2" stroke Panhead would make for a really cool and unique build. Many of the dimensions would cross over from a Knuckle build, but close attention would have to be paid to cylinder length.

Dual carbs would definitely add to the cool factor, but if you are doing it by way of a dual carb manifold, you might want to build it with a single carb initially to get all the other tuning issues ironed out and then make the switch. If you went with a conventional single carb setup, then an S&S E, 42 Mikuni, or even a Harley CV could all work well.

Cam and compression ratio need to be matched for good performance, but I think you should be looking for somewhere around 9:1 for street use along with a cam designed to work with that ratio.

A stock timer with a good performance coil (12 volt)can work very well, as can many of the conversions to electronic ignition. A magneto with the new "rare earth" magnets are also a good option.

The best exhaust would be a two into one with a collector, but you would need to fabricate it yourself since I don't think anyone is building a true performance pipe for Pans or Knucks. Staggered duals will give up some mid range power, but would be easier to find.

Thanks for stopping by to comment!