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.