It appears that the post-1954(?) M20 transmission is common to some more modern boxes as far as internal parts and ratios.

The earlier box has a pretty different tooth pair total (45 for both, instead of 43 for the B34/A10 etc.).
Does anyone have gear tooth counts or ratios for the pre-1954 era? I'm working on an .xls for ratio choices, speed in gears, RPM at speed etc. which will be posted here.

email (option): sales@victorylibrary.com

The M and B Series 'B2' 'late' type gearbox was introduced in 1947 with the rigid B33. It replaced both the earlier 'M1' type fitted to girder fork machines and the 'B1' type box fitted to 45-47 B31s....The B2 box was a development of the earlier M1 box and they shared many components. It was common to both M and B Series bikes built with rigid and plunger frames from that time until 1954, when the B Series bikes got the Swinging Arm type box. (which itself was a further development of the early rigid/plunger A7/A10 gearbox).
M Series bikes retained the B2 box as they remained in plunger frames until the cessation of production of the M21 in 1963.
The B2 (rigid/plunger) box doesn't share any parts with the early A7/A10 box, the B1 box or the Swinging Arm type box. The C11G gearbox is broadly the same as fitted to pre swinging arm rigid/plunger A7/A10 models (but with different casings and a different mainshaft). Both of these share many, but not all parts with the swinging arm type. (The gears, shafts and some other parts are interchangeable). It should also be noted that the C11G box was produced with two alternative gear sets that had different ratios...one the same as the Swinging arm box the other different. There were also competition variants of the M1, B2 and Swinging Arm boxes with different ratios to the standard spec. boxes.....Ian

email (option): ian@wright52.plus.com

Hi Panic,these may help
http://www.petercomley.com/302C%20Gear%20ratios%20rigid%20and%20plunger.pdf]

http://www.a7a10.net/forum/index.php/forum/index.php?topic=2717.0

cheers Rick

email (option): richardholt@rocketmail.com

That's the one, thanks!

email (option): sales@victorylibrary.com