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.
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
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.
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
