So the box was stripped, inspected cleaned etc, the three shafts were set up with minimal endfloat, the selector mechanism was checked, lubed etc for about the fourth time, then the box was rebuilt and replaced in the bike. Result...no difference.

With the box on the bench and being turned by hand, (selector shaft turned by spanner) the gears engage acceptably. With the selector mechanism fitted and the box still on the bench, the gears select although 3-4-3 could be rather hit-and-miss, but with the box in the bike (and filled with oil) 1st is nearly impossible to select, as is neutral from 2nd.
The gearlever still moves OK, but you can feel that the selector shaft is jamming. I'm guessing that possibly due to the drag of the oil plus maybe out-of-tolerance selectors, 2 gears are being engaged at once.
Any ideas anyone?
My next possible solution would be to fit another two-bolt mounting box from an M20 or early B31 (I hope to ride this bike to France at the end of June) so if anyone could tell me the distance apart the two mounting bolts are, plus the width of the lower mounting lugs I'd be much obliged, and if anyone could offer me a box to buy, that would be even better!
Thanks for your time.

email (option): kevin.sandford@ntlworld.com

Kev. First thing i would've done is replace that selector fork that has been rewelded.
Doesn't matter how good it looks, its been damaged & your having selecting problems.

Kevin,

Are you absolutely sure that the clutch is OK. The M20 gearbox is a pig to use whan the clutch is dragging, beause of wear or bad set up.

Regards,
Leon

email (option): leonhop3@planet.nl

email (option): Wow. It seems almost impossible to get the box right.

Leon is wrigth when i have my clutch adjust wrigth it is of the gearbox select smoother Gr >Theo

email (option): oldbikes@kpnmail.nl

i had the same problems couldnt select first when stationary without being brutal and crunching the gears also couldnt select neutral when stationary took one metal one fibre plate from the single spring clutch leaving seven plates and it now selects gears so smooth i keep looking to see if the gearbox is still there

email (option): roger.beck@node6.com

Thanks for the replies. I really wish it was the clutch, but the box is now problematic going through the gears with the engine stopped, ie no clutch involved.
Changing gears by hand from 2nd into neutral can "overshoot" and jam the box (presumably) between neutral and first.
Looking through the hatch the dog clutch "tips" on the layshaft are jammed hard onto the opposing "tips" on the mating gear (ie not engaged just touching hard enough to prevent rotation and subsequent engagement).
When the box was on the bench it was possible to push this dog around until they could engage but space prevents that when in the bike. The only way to unjam it is to rock the bike until the gears engage correctly. Both the gear lever and kick start are "locked" tight when the grears are jammed in this way.
Does that give any clues?
Sorry to keep asking these qustions....

email (option): kevin.sandford@ntlworld.com

They are usually very hard to select gears as you have to turn them a few times before the dogs will align to engage, I've had boxes that will not select any gears on the bench even when I turn it a few times, but when I put them in the lathe and use this to turn them they select very easily (and when they are on the bike) I'd still suspect the clutch

email (option): davmax@ntlworld.com

Selection of gears statically on the bench is always problematic..the selector dog teeth and the teeth on the faces of the gears have to be manually 'lined up' to engage.
The teeth are designed to work correctly with the box running, when inertia serves to pull the teeth together.
The 'overshooting' of the selector dog (in that it goes into gear and then seems to 'bounce' out again) is also a peculiarity of the boxes action on the bench which doesn't occur at operating speeds.
This is the problem with the M20 boxes..how they operate on the bench is not the same as the way they operate in the bike.
If the selector dog teeth are engaging to more or less the full depth of the face dogs in each gear,the selector fork shaft works smoothly with no 'tightness' or resistance to movement and the gaps between the layshaft selector dog teeth and the teeth on 1st and 2nd. gears on the layshaft are equal (when the box is in top gear or neutral) it should work OK when fitted....Ian

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

How do you use a laith to spin the gearbox?

email (option): britool51@hotmail.com

>>>The teeth are designed to work correctly with the box running, when inertia serves to pull the teeth together.<<<<<

Hi Ian, I can understand that, but now the 'box is back in the bike, and spinning the rear wheel or with the engine running, 2nd-neutral seems to overshoot and jam "tooth-to-tooth" (as viewed through the hatch) and neutral-first seems to grind and/or jam too.
I have to take it for the DVLA reg inspection next Monday and it obviously needs to be complete for that, but afterwards I'll strip out the box for another go.
I'm sure it's one of those situations where "someone wot knows" the foibles and fixes of BSA 'boxes could identify the problem instantly, but sadly that ain't me...yet!

email (option): kevin.sandford@ntlworld.com

The best way to put your gearbox in the lathe is to grip the mainshaft in the chuck and with the endcover removed (but the circlip holding the selector mechanism replaced to stop it coming off) then put the revolving centre into the pushrod hole in the other end of the mainshaft, then you can use the gear lever or a pair of grips to select the gears. Rest the toolpost somewhere to stop the box from swinging around. I'd start it off at a very low speed with the clutch and slowly let the clutch out until you are sure it won't jam or lock up. You can then use all the speeds of the lathe to try it's selection at different input speeds, you can actually see what the gears are doing with the inspection cover off.
PLEASE NOTE! BE VERY CAREFUL WITH YOUR FINGERS/HANDS WHEN CHANGING THE GEARS, we don't want any mashed up fingers!!

email (option): davmax@ntlworld.com

Cheers Dave. I will print that off for future use.

That's a nice idea, but I wouldn't want to do it for my problem where the 'box can lock up.
OK for when the gears etc are known to engage as they should, but not so good when there's a possibility that the lathe will try to keep driving a "locked up" box. I can only see damage and heartache then..

email (option): kevin.sandford@ntlworld.com