Ever wonder what it would be like to take a mostly complete 1921 motorcycle and rebuild it as a single cylinder classic Harley racer replica? We are reposting, with permission, this fine first is a series of articles by David Morrill, from his blog www.dlmracing.blogspot.com. We will be following along on his progress in building a 1921 Harley J series street motorcycle as he recreates a blanked off single cylinder classic Harley racer.
Classic Harley Racer: Blanked-Off Twin, Part 1
|1921 Classic Harley SCA Factory Racer #1 from Wheels Through Time Collection|
In the early 1920’s, the engine displacement for short tracks was reduced from 61ci (1000cc) to 30.5ci (500cc), in the interest of safety. As a result of the displacement change, the folks at the Harley-Davidson factory race shop looked around for a way to build a competitive 30.5ci single cylinder race motor. They realized they could remove one cylinder, piston, and rod from their existing 61ci V twin pocket valve race motor and get the 30.5 ci. single cylinder engine they needed.
The pocket valve engines are also referred to as I.O.E. (intake over exhaust) engines having an overhead intake valve and a flathead style exhaust valve below it. This design allowed the intake valve to be quickly changed if a problem arose, which often happened with a one-piece cast iron cylinder and head.
|1921 SCA #1 Blanked Off Twin Racing Engine from Wheels Through Time Collection|
As I examined the motor on the SCA racer, I realized it was not all that dissimilar to the 1921 Harley-Davidson Model J engine that was on my workbench at home. I decided it would be cool to try to build a replica classic Harley racer that folks could see, hear, and touch. Building an exact copy was not in the cards. Harley-Davidson factory racing engines in this period used special early narrow engine cases bearing racing serial numbers. If you could find a set of cases, or any of the ultra rare internal bits that made them so fast, the cost would be too high for my budget. Fortunately, the basic engine layout is very similar to the Model J production motor.
|My 1921 Harley Model J V-Twin Engine Before Conversion to a Blanked Off Single|
When you do a project like this, there is a steep learning curve. First off, the oldest Harley motor I had worked on previously was from the mid 1950s. The engines from the early 1920s, are a very different animal. When I got back to my shop, I tore into my 1921 V twin motor and found the internals were in pretty good shape, considering it’s 90 years old. The one thing that was missing, were the piston rings. This engine was on display in a museum for many years, and the rings were probably removed to keep the cast iron cylinder and piston from rusting together. These early cast iron rings are rare and hard to find. I managed to pick up a junk 1918 Model J motor that just happened to have a good usable piston and rings that would work in my cylinder. Problem solved, or so I thought.
Turned out the piston used a later larger diameter wrist pin, which would require a shift to a later model drilled rod. These rods are slightly longer than the early rods, but that didn’t present a problem. It does raise the compression slightly, by pushing the piston father up the bore, but the engine stroke remains the same. I had a set of these rods stashed away. The crankshaft would have to be disassembled, rebuilt for a single, and then rebalanced.
|Front Cylinder Mock-up|
The rod was shipped to Steve McPhillips at Moroney’s Harley-Davidson in New York. Steve builds Harley XR-750 race engines for many of the top dirt track racers, so it took a while for him to get to my rod. While the crankshaft parts were touring the country, I started mocking up the empty engine cases in my frame. These blanked off motors were built using either a front or rear cylinder, depending on the rider’s preference. I mocked it up both ways, but since the rear cylinder had a nasty wrist pin gouge in the bore, I settled on the front cylinder set-up.
|Modified Generator/Timer Case|
The motor ran better with the smaller carburetor, but still had some problems running properly at full throttle. It was then I realized I had an intake air leak, where the carburetor intake attached to the cylinder head. This is a common problem and it took a little effort to fix. I turned a straight aluminum intake, and rubber mounted it to the cylinder. This is not period correct for a classic Harley racer, but it allows the motor to run well at all throttle setting.