I have Been distracted from the M20 for Some Months by trips to the TT and my Modern Restoration (YamahaFJ1100)
but I looked at BSA Gearbox's This Week I have 3 Gearbox's 2 in Reasonable condition and 1 only good for its worn and Doubtful Parts. The one I intended to use it appears in the Best Condition but looks to have an Odd Mainshaft which is almost 3/4" Shorter Than the Other 2 with No Parallel section for the Sliding Plate this was only discovered after I fitted it to the Engine Plates and Offered up the Inner Chain Case. 2 Questions 1. Can anyone Identify which BSA it belongs to 2. is it simply a matter of replacing the Main Shaft with A M20 Item. See Photos below


This has The shorter Mainshaft measurers 1.433" from the end to the Threaded Shoulder in all other respects it appears a Standard BSA Heavy Gearbox





This has what appears to be the Standard M20 Mainshaft measuring 2.18" from the End to the Threaded Sholder with A 0,72" long parallel section for the Sliding Plate

email (option): grumpy.moran@gmail.com

Hi Ron...I can't give you a definitive answer to that...However, these boxes were used for a number of different models. Presumably, that shaft was for a gearbox used with an engine that had narrower crankcases than the M20...

This same situation occurs with the post 38 'B Series' box..The lightweight models (B24/26) use the box with the short shaft but for use with the B29/30 (with wider cases) a longer clutch sleeve was used to compensate...In the later version of the same box used on the B31, the shaft was lengthened to suit...All this stuff is interchangeable... Ian

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

If i could blow up the picture i might have an opinion? Can you give us a full size photobucket shot of the first pic?

The shorter mainshaft was used on the prewar 4 speed gearboxes from 1932. Like Ian suggests they had a narrower crankcase and the primary chain runs much closer to the final chain.

I think the change in mainshaft length was made when the primary chaincase was changed from the early 30's semi-enclosed one to the full oil-bath type like the M20. There was more space needed for a good oilseal behind the clutch and the only way was the lengthen the shaft. I also think that is the main reason for widening the M20 type crankcase as to get the chain in line again.

Also this (unwanted) greater overhang of the primary chain gives much more force on the gearbox mainshaft and bearing and that might be the reason of the often excessive wear of the long bronze bush which supports the mainshaft. After all, the original gearbox was not designed with this longer shaft in the first place.

Any thoughts on this theory very welcome, it is interesting to find out the reasons for the various design changes during the production.

So Ron, The gearbox in the first picture looks like a common M20 but indeed has the early mainshaft and the gearbox on the second picture is a prewar one, is has a longer oil-filler inlet and some more detailing of the case. Probably from the late thirties as it has the longer shaft used in combination with the oilbath-primary case.

Regards, Michiel / NL

Thanks guys for the info luckily I have a spare M20 main shaft should I want to use this box or failing that sell it off as a M20 box

Cheers

Ron FTLDU

email (option): Grumpy.moran@gmail.com

Michiel, i am like you very interested in changes & why they were implimented. Not sure if you meant in your post that the M20 wider (1939) cases had a different position drive train ....but the 37 & 38 narrow M20 engines used the same length gearbox mainshaft as the later ones. the outer drive side bearing was "outtigged" but the primary chain line wasn't moved out.

Hi Fred...The extra width of the post 38 redesign, M20 crankcases was all on the timing side and resulted from the introduction of the air gap behind the tappet chest....Ian

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

OK, Interesting Fred. Do they have a different chaincase which take less space than the later ones??

I also had a closer look at my spares and found a gearbox with a single spring clutch attached where the primary chain runs much closer (appr. 18mm!!) to the gearbox than the common M20 one. The difference must be the clutch sleeve.




So it could be this short type of clutch sleeve (or a wet 6 spring version)that was used in combination with the narrow 37/38 engines?

Ian, wasn't it so that the M20 cases were also changed on the driveside? I don't know the exact early construction but thought the ballbearing was added or at least placed much further outside ??

Regards, Michiel