The Rupert Ratio Manual

Rupert Ratio Unit Single Engine Manual

"I recommend this book ..." The Star (BSA Owners Club magazine)

"Anyone embarking on a rebuild of a BSA unit single will get a wealth of tips from the Rupert Ratio Manual." Classic Bike

"If you have a sick or senile BSA, try this book." VMCC Magazine

"Bottom line is that if you are even thinking of wrenching on your BSA Unit Single, you need this book." BSA Southern California Magazine

Armed with this book even the least experienced 'spanner man' will be able to not only rebuild a BSA Unit Single, but make it go better and more reliably than even BSA thought possible! This book quite literally takes the BSA (and Triumph) Unit Single engine apart, then it shows how to rebuild the engine correctly. The book includes wiring diagrams, complete specifications, hundreds of illustrations, a wealth of invaluable advice and tuning tips to make your unit single perform!

The Rupert Ratio Unit Single Engine Manual will also be of interest to all BSA enthusiasts since it charts the development of one of the last and most successful machines to come out of Small Heath. Long after the factory closed Unit Singles were still a competitive force in trials and motocross riding and many are still used for that purpose today.

Rupert Ratio is a long time contributor to the journal of the BSA Owners Club, The Star, on technical matters relating to BSA Motorcycles. He is also one of the many 'Agony Uncles' for membes of the club, i.e. a technical consultant who can offer advice and assistance to club memebers when their beloved Beesas are beset by mechanical afflictions. He carries out the latter role under his real name, Dave Smith, who is a design technology teacher by profession.

  • Introduction
  • The Unit Single Saga
  • Unit single quick stripdown guide
  • Preparation of aluminium castings
  • Lubrication system
  • Main bearings
  • Crankshaft and conrod assembly
  • Timing side
  • Gearbox
  • Kick start mechanism and outer timing cover
  • Primary drive
  • Cylinder and piston
  • Pushrods
  • Cylinder head
  • Rockerbox
  • Engine breathing
  • Electrical system
  • Carburation
  • Life with a B40
  • Specifications
  • Exploded engine drawings
  • Appendix: Rupert Ratio engine stand
  • Index


First published by Panther Publishing in 2000.

My first bike at the age of seventeen was a BSA C15 - I still have one now. As an extremely enthusiastic, but slightly optimistic mechanic, I have spent many hours over the years, sweating over a partially dismembered engine, trying to spot the difference between the way the manual said things should be and the way I left them last time I was in there. These fumblings were based on a small amount of common sense, a few badly photocopied BSA service sheets, and a lot of trust in the Haynes manual.

If only I knew then what I know now…. This book written by Rupert Ratio (aka Dave Smith) is the ultimate book if you fancy (or are being forced to undertake) any mechanical work on any of the range of BSA unit singles, is an absolute necessity. It covers the BSA C15, B40, B25, B44 and B50 models with plenty of explanation as to where each of the models differ.

The author is the technical consultant to the BSA Owners Club for these models and is able to write this book based on many years of personal experience. This is reflected in the numerous "handy hints" that are used throughout the book. These have proved invaluable to me as they offer a real world solution to many of the minor obstacles posed. Many of them are not specific to BSAs, but cover many general topics such as carburettor tuning, making your bike oiltight, and repairing an alternator.

Many BSA Service sheets are contained within the book, although somewhat surprisingly they were perhaps the least useful aspect of the book.

It is very hard to fault this book in any area. It contains many photographs and diagrams that illustrate all of the work described, but it is not just a workshop manual. It is also a good read for anyone interested in the history of BSA

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