The 45” W-Series flathead (1937-73 W, WL, WLA, WLC, G, etc.) is the smallest and slowest V-twin that Harley-Davidson has produced in the last 70 years. Its low compression, obsolete port and valve positions, limited displacement, small carburetor and mild cam timing make for reliable, but un-exciting performance.
    There are many methods that will substantially improve performance, by the usual addition of more compression, bigger cams, larger carburetor, etc. but their success is limited by the basic side-valve design. Regardless of how much money is spent, how many rare factory racing parts are installed, and how many hours are spent fine tuning, the motor will never be competitive with 45” over-head valve engines.

    However, over 30 years ago a famous 45 enthusiast named Randy Smith noticed many similarities between the 45 and Sportster motors. After careful planning, the first known “45 Magnum” engine was created by grafting 900cc Sportster cylinders and heads to the 45 crankcases. This booklet is the result of many years of research and investigation (including a visit in 1973 to the original Magnum engine in Long Beach, California).
    Here’s some large pictures of a Magnum motor in various stages of construction (courtesy of enigmas): .
    I have attempted to give as much advice, commentary, observations and suggestions as possible to offer the greatest amount of assistance to those who would like to try this ambitious project, but who have no specific information to begin with.
    The Magnum is definitely not for novices; both heli-arc welding and complex machine work are required, as well as permanent & extensive modifications to the frame, but no expensive custom-made parts or commercial speed equipment. With a 45 motor and a 1957-71 900cc Sportster top end, you have most of what you’ll need.
    Although comprehensive and informative, this booklet is not a manual with specific instructions detailing how to accomplish this - there are too many variables & choices. I decided not to just add pages from the Sportster and 45 service manuals, diagrams of stock motors, pictures of custom bikes, etc. - you already have that information. The booklet is fairly modest (15,000 words) because it only contains information you'd need for the conversion. It is the best information available, and it does offer many helpful hints, calculations, illustrations, comments, observations, data, etc. on this complex subject.
     If you’ve considered doing this, you should read this before you make any plans, or spend any money. The information applies specifically to Harley-Davidson 45” 1937-73 flathead motors.
    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 one new idea, doesn’t that make this purchase worth while? The price is a small fraction of what you’ll have invested in the motor - plan it more intelligently, and get better results.
    This information is not included in any other of my other booklets; click here for other related titles: . This booklet is recently revised and enlarged (September 2008) and now is 23 pages, includes 38 black & white illustrations & diagrams and data tables. I’ve included metric equivalents for most fractional and decimal units for the convenience of builders using other systems. Subjects covered include:

  »  Comparison between 45 & Sportster components
  »  List of required parts for the conversion
  »  Welding preparation
  »  Bridgeport machine work
  »  Methods of attaching the Sportster cylinders & heads
  »  Choosing the best compression ratio
  »  Improving combustion chamber efficiency
  »  Selecting, modifying, re-timing and installing cams
  »  Flywheel weight, how it affects performance
  »  Modifying the 45 tappets & blocks
  »  Ignition systems: types, installation & tuning
  »  Exhaust systems: choices, fabrication, installation & tuning
  »  Frame modification for Sportster rocker box clearance
  »  Other useful comments about the chassis, etc.

    Please note: this is not a coffee table book, it is intended to be a research tool for those who seriously want to perform this complex conversion. It is not a repair manual. If your bike won’t start, float bowl leaks, spark plugs foul, &c. my booklet will not help you. It does not contain information on tune-ups, or explain how to remove, disassemble, clean or restore the engine. If you do not already have this information do not buy this booklet. If your engine is stock and you’re pleased with how it runs now, this booklet is not going to be of much use, although you might find it interesting.

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