BSA Gearbox question

[zitman]

I realise this is off topic but I have a problem with a gearbox on an XB32.  As far as I can see it is the same as the gearbox on my old A7 Plunger so hope you can help before I set fire to the thing!

On the move it selects 1st 2nd and 3rd ok, when changing to 4th it won't go in.  The gear lever has no downward movement whatsoever, it is solid.  But when i try to change from 3rd to 4th without the clutch it slots right in, going back down the box 4th to 3rd goes in fine but the same problem arises when trying to go from 3rd to second.  Again if i change without the clutch is goes right in sweet as you like.

This also is the same without the engine running, no clutch and it snicks through the gears fine hold the clutch in and the above problems occur.  I have put a cable tie around the clutch lever, it goes into 3rd but when I put pressure on the gear lever there is no movement.  If I maintain the pressure and back off the clutch pushrod adjustment until the clutch has effectively released the clutch it engages 4th right away with minimal force.

I have taken the outer cover off and all of the gubbins is moving freely and correctly adjusted, the mainshaft (where the pushrod goes through) has some endfloat.  I removed the inner cover and found that a washer was missing that sits between the nut on the end of the mainshaft and the inner case.  I fitted a washer of 1.5mm and put it back together, no endfloat, but my problem is the same but this time without the clutch connected as i was turning the selector with the outer cover off.

I reduced the washer size to 1.00mm this gave me some end float but i am back where i started, unable to select 4th with the clutch lever pulled in.

Perhaps i don't have enough endfloat?  I figure it is a clearance problem?  The inside of the box is pristine.

Any ideas PLEASE because I am completely flummoxed.

Z

[BSA_54A10]

Check the mainshaft.
Whiles the actual internals of all BSA preunit boxes are very similar and the gears are interchangeable there arel sufficient differences outside the innerplate ( sandwich plate to some) to give you grief.
The only difference inside the box between clutch and no clutch is that the mainshaft gets pushed through the box by the weight of the clutch springs.
This can cause the mainshaft to move on early, light weight and war boxes.
A bit latter on BSA fitted a retaining clip on the kick start side mainshaft bearing because it was found to be moving.
Now as yours seems to be moving quite freely then I would hazard a guess that the bearing is either very loose or has worn too much.
The other sign of this is the kick start not returning cleanly ( grinding ) or sticking down completely.

The mainshaft is retained by the nut on the end of the kick start ratchet and has some spacers between the bearing and the plate to position the shaft in the correct place place in the box for the forks to be able to engage and disengage the gears cleanly.
Pull it out and check that the bearing is tight up against these spacers then grab the main shaft and try a push pull test to see if there is any movement.

OTOH if you are a contortionist you can pull the inspection plate off the box poke a mirror into there and watch the clearances between the gears and the forks. Get two others to help you ( excuse for a couple of beers ) and while one trys to turn the rear wheel at a constant speed the other will need to pull and release the clutch lever. The space between the forks and the gears should be even and not change without the gear change cam rotating.

As the box changes "normally" I will assume that the ratchet plate and pauls are still in good condition with nice sharp ( ish) ends and that the return springs are in good condition.