Thursday, December 6, 2012

Building a 1921 Harley-Davidson Race Engine - Episode #8

By: David L. Morrill
@ Mototique

Updated - February 13, 2014

This project started in 2010, when I visited the Wheels Through Time Museum in Maggie Valley, NC. The Motorcycle CannonBall Run cross country race, for pre-1916 bikes, stopped there. The visit allowed me to check out both the W.T.T. collection and the CannonBall entrants. The collection at Wheels Through Time is one of the finest collections of early motorcycles and memorabilia in the country.  It's a must visit for any motorcycle enthusiast:

While examining their collection, I came across a very rare early racer. The bike in question is a 1921 Harley Davidson SCA (Single Cylinder Alcohol) serial #1 racer. Looking at this old racer, in as raced condition, most would ignore it in favor of the bright and shiny restored bikes. These rusty relics are often worth way more than the bright and shiny restored bikes. As they say, it's only original once!

1921 Harley Davidson SCA Factory Racer #1
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.

1922 Harley-Davidson SCA Racer
Glenn Bator Collection

The pocket valve engines are also referred to as I.O.E. (intake over exhaust) engines having an overhead intake valve and a flat head 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
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 and build a replica 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.

Crankshaft Modified for a Single

I contacted my friend Jim Haubert, who worked for the Harley-Davidson factory, and asked him to modify my crankshaft. We decided to use the rear forked connecting rod, and the race from the front rod, to contain the center roller bearings in the big end. When Jim got the replacement rod, he realized the pressed in big end bearing races needed to be replaced. He didn't have a fixture to safely replace the bearing races.

Front Cylinder Mock up
The rod was shipped to Steve McPhillips at Moroney's Harley-Davidson in New York. Steve builds 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 nice wrist pin gouge in the bore, I settled on the front cylinder set up.
When the crankshaft returned from Jim's shop, final assembly was pretty easy. I modified a set of rockers to just work on the front cylinder cam lobes, and and made a plate to cover the missing cylinder. George Hood sent me one of his prototype rocker towers, with a longer rocker. This setup lets the engine breath a little better. The generator was modified, eliminating the armature. The timer points cam was modified to fire only on the front cylinder. The ignition is now powered by small 6 volt battery firing through a modern coil.
Modified Generator/Timer Case
Within a few weeks, I the engine installed in a street frame, and fired it up for the first time:

YouTube video of the initial start up:

The next problem was the carburetor. I originally used a modified V twin intake, which had the carburetor come out on the right side of the engine. The original Model J  twin cylinder engine used a one inch bore Schebler carburetor. After the initial runs, I realized it needed to be changed to a smaller 3/4" bore Schebler. I found a guy, who had one, and we worked a trade.

Modified Intake & Ignition

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 attaches 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, but it allowed the motor to run better at all throttle setting.

I recently switched to a Linkert M741-1 carburetor from and Indian Jr. Scout. The Linkert features separate high and low speed adjustable needles, where the Schebler had only a single needle to adjust the fuel mixture at all throttle settings. This was a great improvement, that allows the engine to run well at both high, and low speed throttle setting.

Linkert 741-1 Carburetor with modified Rubber Mounted Intake

Stay tuned for Part 2
Assembling a Keystone Racing Chassis


  1. Hi there, awesome site. I thought the topics you posted on were very interesting. I tried to add your RSS to my feed reader and it a few. take a look at it, hopefully I can add you and follow...

    Dade Dodge

  2. David,
    Email me if you would.
    Lee 'Onion' George
    aredandgold at msn

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  4. Glad you enjoyed it Powesports! I'm starting to work on part 2, which will explain the process in recreating a 1920s racing chassis.

    Deadly Dave

  5. Nice post.
    I like the way you start and then conclude your thoughts. Thanks for this nice information. I really appreciate your work, keep it up.
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  6. Thanks Stevan,

    It's always good to get positive feedback. Story telling is considered an art for here in the South, and it's sometimes a high bar to meet!

    When I started writing, I decided to follow the KISS method, Keep It Simple Stupid. My niece is a much more gifted writer, and she get paid for her work, but I decided to write my stories, as if I was telling a friend a story. As an old motorcycle racer, these early bikes, and the fearless folks that raced them are my passion.

    To me, sharing their stories, is a privilege, and I hope I do them justice. When I start a story, I introduce it and hopefully gain the readers attention, tell the story and share photos to hold their attention, and then wrap up with my final thoughts. It works for me, and I hope it work for mu readers.

    I promise, as soon as I get down with my current racing story, I will get to work on part 2 of my 21 Racer story! Hope you can one day make it to the Barber Vintage Fest and see it run. I get a big kick out of sharing it with others!

  7. Thanks Stevan,

    Telling the stories of these early bikes, and the fearless folks that raced them is my passion!

    We have a rich history of story telling in the South, and I try to write my stories as I was telling them to a friend. It seems to work for me.

    I'm working on another early Birmingham racing story, and when it's done, I'll get to work on the part 2 story on my 21 racer.

    I hope you can make it to the Barber Vintage Fest one day and see it run. I love to see folks reactions to a 92 year old race engine running in anger!


    Deadly Dave

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  9. Thank you Ella, I'm glad you enjoyed my blog, and I hope you learned something new! I try to put up at least one new story a month, and I update the older ones with new photos, facts, etc. as they become available.

    Deadly Dave

  10. An impressive post, I just gave this to a colleague who is doing a little analysis on this topic. And he is very happy and thanking me for finding it. But all thanks to you for writing in such simple words. Big thumb up for this blog post!
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  11. Hi buddy, your blog’s design is simple and clean and i like it. Your blog posts are superb. Please keep them coming.
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  12. Thank you Stevan. Glad you enjoy the blog!

    Deadly Dave