If you own (or plan to own) one of these great bikes, you already know that the 45 is reliable and has that timeless classic styling, but is awkward to ride with the original foot clutch and tank-mounted shifter. The original 3 speed transmission doesn't accelerate well, and you frequently find yourself looking for another gear! Unfortunately, there is no 4-speed for the 45 (the big twin gearbox doesn’t work, and too big to fit).
    There is hope! The 4-speed transmission from many older (non-unit) British motorcycles (Triumph, Norton, BSA, Royal Enfield, etc.) can be adapted for use in a 45 solo without cutting or welding the frame; no permanent modifications to the bike are needed (the bike can be put back to stock if you wish). The hand-operated clutch and foot pedal gear-shift lever feel “more comfortable” to most riders, but the real advantages are:  »  More transmission ratios (twice as many intermediate gears, 2 instead of 1).
  »  Potentially greater range between 1st gear and high gear - better 1st gear acceleration.
  »  Smaller RPM drop when shifting into high gear - the motor doesn’t “die”.
  »  More sprocket choices.
  »  Higher top speed potential.     Please note: a 4-speed is not overdrive and will not increase your top speed unless the wheel or sprocket sizes are changed. High gear in all H-D and British transmissions is 1:1 regardless of how many gears they have.     Although comprehensive and informative, this booklet is not a manual with specific instructions detailing how to accomplish this - there are too many variables & choices. It is the best information available, and it does offer many helpful hints, calculations, illustrations, comments, observations, data, etc. on this complex subject.
    Here’s some large pictures of a BSA M21 trasmission installation (courtesy of enigmas): .
    If you’ve considered doing this, you should read this before you make any plans, or spend any money. By the way - the conversion to a British 4-speed may be cheaper than buying and rebuilding a 45 transmission & clutch assembly. The information applies specifically to Harley-Davidson 45 Model flathead solo 1941-1952, although most of the information is useful for both older model solos (1937-40), and Servi-Car (trike) 1937-1973. An Indian Scout owner will find some interesting ideas here, too (although this conversion is not addressed). This information is not reprinted from other articles, magazines, or factory literature and is not available anywhere else, and not included in any of my other books; click here for other related titles: .
    I guarantee that you’ll find information in this booklet you’ve never seen before, and that will help you plan and complete your transmission conversion.
    Consider this: if this booklet saves you from making even a single mistake or bad purchase, or gives you one new idea, doesn’t that make this purchase worth while? The price is a small fraction of what you will invest in the transmission - plan more intelligently, and get better results. This booklet is 28 pages (over 17,000 words) soft-bound in paperback, includes 25 black & white diagrams & illustrations, plus 28 data tables. Subjects covered include:   »  Possible British transmission choices, by brand and model (over 20 choices listed).
  »  Some hints on identifying an unknown transmission, including photos.
  »  Checking out a used transmission - what to look for, what to bring with you, what questions to ask.
  »  How to fabricate a motor sprocket compatible with your 45 engine and British primary drive.
  »  Primary chain types - there are more than you think, how to tell them apart.
  »  What internal gear ratios (i.e., close, wide, etc.) are available for British transmissions, including the 4, 5 &
      6-speed speed factory and aftermarket gearsets.
  »  How to choose an internal gear ratio set, based on your purpose and engine mods.
  »  Selecting sprocket sizes for best acceleration or top speed.
  »  Making an adjustable bracket to mount the transmission.
  »  What to do about controls (shifter, brake, etc.).
  »  List of suggested parts for the conversion.
    Also included is an update which contains useful information about installing the 5-speed gearset from the 1973-* later Triumph T120V 650 & T140V 750 twins, and T150V, T160 & A75 750 triples into the Triumph pre-unit box.

    I guarantee that you’ll find information in this booklet you’ve never seen before, and that will help you plan and complete your project.
    Consider this: if this booklet saves you from making even a single mistake or bad purchase, or gives you new idea, doesn’t that make this purchase worth while? The price is a very small fraction of what you’ll spend on the project - plan it more intelligently, and get better results.

    Please note: this is not a coffee table book, it is intended to be a research tool for those who seriously want to improve their 45 motorcycles. It is not a repair manual. It does not explain how to remove, disassemble, clean or restore the transmission. If you do not already have this information do not buy this booklet. If your transmission is stock and you’re pleased with how it works now, this booklet is not going to be of much use, although you might find it interesting.