B31/M20 Gearbox

[martinh10]

My 1950-ish plunger B31 bitsa has M20 stamped on the inner gearbox casting. My B-Series spares book gives me the number of teeth on each gear but I cannot find the same information for M-Series (I should have counted them when I rebuilt it - dammit). Are they the same with different gearing requirements taken care of by the engine/clutch/gearbox/rear wheel sprockets?

[Rog1]

I believe it is the same gearbox as in my M33. Top gear has to be the same overall ratio (given the same primary and final drive sprockets) because of the design of these gearboxes, but 1 to 3 are lower.

Both final drive sprockets and the clutch sprocket whether M or B all have the same number of teeth. Solo gearing for me on a 20T engine sprocket (This is all from the Haynes manual, but I'm sure I've seen it elsewhere too) is:
4   4.75:1
3   6.25:1
2   9.77:1
1 14.15:1

Sidecar gearing with a 17T engine sprocket gives me:
4   5.59:1
3   7.37:1
2  11.5:1
1  16.75:1

Standard B31 gearing, (again from the Haynes manual) with a 17T engine sprocket is:
4   5.59:1
3   6.77:1
2   9.86:1
1  14.42:1

So your speed in first gear is 14.42/16.75 = 0.86 of the speed it would be with the B series box; in first gear you'll be going at (say) 17.2mph instead of 20 mph. You'll have probably already changed up to 2nd by this point, but the maths is easy...

[martinh10]

Thanks Rog - finally got my head round that. That strongly suggests that I have the M20 internals in my box as first and second seem way under-geared whereas third and fourth seem about right on standard B31 sprockets. When I strip it I can confirm this if the number of teeth on the cogs are not on line with my B31 data but if anyone has the numbers for an M20 box I would appreciate it.

[Rog1]

As far as I can work out from various diagrams, the number of teeth on the gear pairs in an M series box is, (from left to right, layshaft first)

17 & 28
20 & 25
25 & 20
29 & 16

I don't know which gear pair does what, but I suppose you could work it out.

I wouldn't want the gap between 3rd and top to be any wider than it is, but as this is my first pre-unit single, I have nothing to compare the gear ratios with.

[martinh10]

I'm afraid I did not pay close enough attention to your original reply Rog. The M registered but not the 33, I was thinking 20. Your gearbox will be the same as a B33 which has the same cogs as a B31. So if there's any M20 gearbox experts out there .......

[Rog1]

I'm afraid I did not pay close enough attention to your original reply Rog. The M registered but not the 33, I was thinking 20. Your gearbox will be the same as a B33 which has the same cogs as a B31. So if there's any M20 gearbox experts out there .......

Martin,
as far as I know the same gearbox was fitted to both the M20, M21 and the M33; or at least the ratios are all the same. I checked my M series parts book where it lists only one gearbox, and I also looked at various parts books and handbooks in the member's section of the BSAOC website. My M33 is essentially an M20 with an OHV 'B' series engine instead of the side valve lump of the M20; even my frame number starts with 'M20"!

If anyone knows any different, please let us know.

However, it seems that early 'B' series boxes looked very similar (if not the same) as the 'B' series box, so all I can say is my bike is supposed to have an 'M' series box; what it actually has I don't know. That'll be the can of worms I'll open when I find myself needing to do deep gearbox work!

[Calum]

I rebuilt my plunger B31 box recently and I bought a box for spares (many for the good selector forks) but I did end up using a gear or two out of it. It was stamped M20 on the casing but came disassembled so I can't guarantee it was all correct. I'm pretty sure the gear ratios were identical.

[martinh10]

Rog/Calum, thanks for your inputs. I'll assume they are the same cogs for the time being unless I hear otherwise.