Hi
Now that the engine build is completed - I am preparing parts for gearbox build - again mostly nos - so I am hoping for a good result & a straightforward assembly -
not the first time I have been into a box / but the first time in rebuilding one completely
Looking at the selector forks & groove pins - I know there were 2 types in service - but today when digging stuff out - I came across 3 different types
1/ narrow selector grooves & pins - with a tag nos B31/33 ?
2/ wide selector grooves & wider heads on pins - nos parts
3/ wide selector grooves & wider heads but thinner heads equipped with rollers underneath - from a used used box
I am using no 2 as this was the later war box type with allegedly smoother shifting
My question - no 3 assembly - was this ever seen in an m20
Br
Jo’b
Re: Bsa M20 Gearbox - control shaft, selectors & pins
I must admit I haven't mentally recorded what was fitted to every gearbox I've stripped and there have been many...
I know exactly what you are describing with the '3rd type' and have seen them...As I have personally changed the selector assemblies in a number of boxes from one type to another there is always the possibility that what is found is not original fitment anyway...
I guess in depth study of parts lists is the only resolution to your question, though I suspect pre or post war fitment is the most likely for 'wide track' components...This is another component that BSA didn't use consistently it seems...Regarding your own box I'd go with the 'wide track' selector forks but with either type, if they are not NOS, ensure that the amount of track wear, particularly where the track changes direction, is minimal in terms of clearance to the pins...
You also haven't mentioned selector dogs and the corresponding teeth on the faces of the gears...These were also modified to improve engagement and the later type would be beneficial...It should be noted both types can be 'mixed' in the same box....Ian
Re: Bsa M20 Gearbox - control shaft, selectors & pins
AFAIK the roller type were a pre-WWII idea
The slot is actually narrower than the wide pin type.
I had this type fitted in the bike when I got it but I was the third owner and the original owner rode it as daily transport for 40 years and it wwas his only transport as he was a single man all his life .
It is by far the smoothest shifter of any of the horseshoe boxes I have used, even when thoroughly nackers it still changed well
I currently have a narrow type fitted & it is fine on the up changes but will happily go from top through to neutral bypassing 3rd & 2nd all together .
Re: Bsa M20 Gearbox - control shaft, selectors & pins
Hi Wm20
I have my suspicion this came from a gearbox on a km20 supplied to the irish army in 1939 but until I started preparing the box & components to rebuild it - I never noted it
With a roller mechanism - it would have been a smooth change - I also found these roller forks in some Japanese boxes I worked on in the past
