more Gear box issues BSA B31

[Barthol_18]

Hi,

after reinstalling my B31 Gearbox ( All gear engaged on the bench) I found that it was impossible to engage 3rd. gear when driving the bike 1st. 2nd. and 4th. were fine .  Taking the box apart there seem to be a lot of wear on both DOG Clutch wheel as well as on the selector forks( See attached photos)
I giuess this must be an issue?

how do I remove the selector forks? the guide pegs do not seem to be threaded into the selector shaft.
Are they supposed to be drifted( knocked) out?

BR
Kim

[Jim S]

The guide pegs are tapered pins. I think you can drive them out via a hole or slot in the back of the fork. I had the same problem as you about 8 years ago.

[Barthol_18]

Thanks a lot Jim, I will try that

[Calum]

I had the same issue on 3rd gear on mine when I bought it. It would drop in and out of gear, especially on the overrun. I can only assume it was caused by excessive end float in the shafts. I rebuilt it and sourced a spare selector dog and forks from a donor gearbox, and all is good now.

From memory the pegs are peened over and you'll probably have to destroy the back 'peen' to remove them. Fairly sure they're case hardened as well so replacement isn't as simple as quickly turning one up on the lathe. I may be remembering incorrectly here so my apologies if so.

In all honesty I'd look for a whole fork and shaft assembly as they're not particularly hard to come by. If you do get a new fork do be aware of the different peg diameters. The later ones have a revised 'track' and increased peg diameter to improve shifting I believe.

[Jim S]

When I rebuilt my gearbox I replaced the selector dog as Calum did, but I salvaged the original fork with a weld repair and had the faces ground flat.

When rebuilding, it is important that the fork faces are not touching the selector dogs after any gear is selected. I did many interations of stuffing the assembly into the housing and checking for fork rubs. I used a Dremel grinder on the fork faces to make minor adjustments.

For shifter shaft end float, I measured 0.016in at rebuild. I did not do any correction to this. I'm not sure what excessive end float would be but in this forum other people have made corrections when 0.030 end float was measured.

Since the gearbox rebuild I have put 8,000 miles on this 1949 B33 and it is still shifting well. 

Jim

[scifi]

Hi Barthol, sorry to say, but that Dog Gear is scrap.  The edges are rounded out, and most likely engage with gears that are also worn out.  The corners of the Dogs should be square, as you can see from the unworn part in the third photo.
I don't think anyone would re-machine these on a milling machine.  So I hope you can source new parts.
The Selector Forks are worn because of the sideways forces the Dog Gear has been putting on them.

[limeyrob]

BSA gears are very well made and hard, the only reasonable option after 70 years is to get a replacement.  There's a lot or good used parts about if you can't find new old stock.

[Mark Cook]

This should help.
https://pesltd.uk/?product=24-4393

There are two versions of these, the later version having a pitched face.
They all run together, if all the gears are the later version it will change gear better at higher speeds.
One thing to watch out for is inaccurate machining.  The groove if often biased to one side or the other.
Selectively fitting can improve the alignment.

Mark