I second the need for correct shimming. I recently rebuilt my gearbox and didn't leave enough end float on one shaft and it was impossible to engage 4th on the road. Only a few thou but critical.
Strangely it would engage all gears with the gearbox on the bench.
The inspection cover makes identifying problems much easier.
It seems odd that in WWII BSA was making a gearbox then needed to be shimmed? Would it not have been better to make all the parts the same dimension so no shimming was required? A Triumph, Velocette or any other WD bike did not need shimming and no inspection cover was required. Needing to shim increases labor costs and make rebuilding a gearbox in the field difficult. Even after the war, BSA did not fix the problem.
What were the engineers who designed the gearbox thinking or were the gearboxes made on completely clapped out machine tools?
I don't think they were shimmed when they were new, problems caused by 75 years of wear aren't a design flaw, and the fact that you can get them back to original spec with shimming is a bonus.
Rob
email (option): robmiller11(a)yahoo.co.uk
Bruce, I'm shocked that you are on this website with so little knowledge of the history of the time.
It takes 5 years to train as a mechanical engineer. A lot more to be "time served" let alone "skilled"
Factories were told to stop what they were doing and make parts for the war effort.
Precision measuring equipment was expensive and rare. Go NoGo gauges were used for many operations.
Operators or even companies would have little idea of what the part was for or how it worked.
Remember the machines were often driven by flat belt. To change gear you hit the belt with a stick.
In defence now, it's still the same for many parts.
Everything was selectively assembled.
The BSA and military assembly instructions are excellent if parts were made to drawing.
This is why I wrote a guide to overcome the problems.
It's just the same as any skilled "Fitter" would have done.
Mark
email (option): pes.sales@btconnect.com
Mark,
You are making excuses for BSA. A WD Triumph, Matchless or Ariel made at the same time as the BSA has no shims in the gearbox at all. Somehow they figured a way to make the parts to the required tolerance. BSA had been making this gearbox years before the war and certainly could have solved the problem.
As the war was on and the need to make bikes as fast and cheap as possible, having to shim is a big waste of time and it increases labor costs. The need to shim also allows new and inexperienced workers to put it together wrong. Although they had no CNC machines, BSA still had things like swiss lathes that can make the same part to the required tolerance all day long without an operator.
The fact that some BSA engineer did not ask "how come a Triumph has no shims in its gearbox and ours is full of shims and takes twice as long to assemble" makes no sense?
I overhauled a Burman gearbox a few months ago from a 1938 AJS. No shims anywhere.
Although retired now I worked in manufacturing for many years. If an engineer designed something that needed to be shimmed to make it work, they would be shown the door.
Bruce
I'm not sure that many people do shim BSA gearboxes, I suspect they are in the minority, they just happen to post here about it, I never have and probably never will, most M20 owners clunk on with a mixture of worn out and NOS parts the same as everyone else.
The inspection cover is a brilliant idea, especially if you are opperating in a dusty or sandy enviroment where a fault can be identified without pulling the entire contents of the gearbox out, and is it that wastfull, it replaces a piece of valuable alluminium with steel after all.
Rob
email (option): robmiller11(a)yahoo.co.uk
The M20 gearbox was a design that dated from well before the war and was originally a hand change gearbox that a footchange mechanism was added to...The Norton had a similarly antiquated design with their box which also dated from well pre war...Both shared the bad features of these early boxes...Poor gearchange mechanism design, overly heavy gears with too much inertia, multiple opprtunities for 'lost movement' etc. and in BSAs case over complicated assembly procedures..
In the period leading up to the war advances were being made in gearbox design, notably the 'camplate' design used on Triumphs from that period...These gearboxes were the designs that became the standard after the war...
Triumph, for example, were more advanced in this area and it wasn't until the postwar rigid A7 in 1948 that BSA 'caught up'...This early version of a BSA 'modern box', perhaps unsurprisingly, closely resembled the Triumph box in many of its features...Used on all the rigid and plunger twin models it wasn't altered again until the introduction of the swinging arm models, for both single and twins, in 1954...
Post war singles struggled on with the earlier type boxes in various forms until this date and it can only be assumed BSA didn't want to re engineer the cycle parts (and crankcases) on these models to take the new (1948) gearbox design from the 'semi unit' Twins...
There is no doubt that BSA were slow and well behind others in improving the gearboxes across the range but refering back to wartime comparisons the fact is BSA and Norton were simply at different stages of developement to other manufacturers....In reality most manufacturers aren't constant innovators and they don't all move as one, rather they follow the direction of others who are leading the way...In this case there's no disputing BSA were following...
Now, lets have a discussion about comparisons between the cam and followers designs of wartime engines...:laughing: ...Ian
email (option): ian@wright52.plus.com
I had a problem with the clutch not working, it simply did not free upp when I pulled in the handle. Turned out to be that the supplier of parts had sent my the wrong rollers for the clutch basket, I think 1/4 X 5/16 instead of 1/4 X 1/4 (but I may remember the numbers wrong). Once the correct rollers were fitted it worked just fine. I have the single spring clutch.
Best regards,
Simon
Yes Simon they should be 1/4 x 1/4. Not as simple with Triumph's though! Suppliers will often send you 1/4 x 1/4, when in fact the Triumph rollers are about 12 thou (.012") shorter in length. This caused me a lot of problems many years ago with my first Triumph. Ron
email (option): ronpier@talk21.com