Help needed with BA gearbox rebuild

[paul.jameson]

The thread is 1/2" x 20tpi, probably whitworth form so a UNF x 20 tpi won't quite fit, but very nearly.

You need a special nut for that end of the mainshaft which is effectively a locking nut (commonly known as a half nut, although generally thicker than half a normal nut) with the threads on the inside (ie clutch side of the nut) countersunk.

[Alan G]

Thanks Paul. I have the special nut, but it doesn't fit as the threads have been 'modified' with a wrong nut forced on. Searching for a die to rectify, which is why I was checking correct size.

[nevhunter]

Be careful it doesn't end up a Loose fit. 20. Tpi is the coarse version of the BS Cycle threads. 26/20 whitworth form..Nev

[Alan G]

Nev, not sure I understand... are you saying bsw 1/2 20tpi is the wrong die?

I am going to borrow a (hopefully comprehensive) set of taps and dies from my local classics club, so hope to use the correct die, not one that is 'fairly close'.

[Roger Gwynn]

The thread angles of Whitworth, UNF and BSC are different by 5 degrees, in practise you can get away with either unless your thread is a precision one and a very good fit, which is to won't be as it is already damaged and 80 years old.

[Mick D]

Hi

Does anyone know what the thread actually is? I've always assumed it was BSCY / CEI

Regards Mick

[nevhunter]

It Won't be NF and a die has a lead in that won't get to the shoulder. Careful use of a thread file or Putting it between centres and chasing the thread carefully will do it, but what you take off will be Miniscule. You don't want a Loose thread. This is Probably the Most critical thread on the Bike. Nev

[Julian Murphy]

All depends on how damaged the thread actually is.

Start with a new nut of the correct type/thread.

I would use a needle file or files to make the correct 'adjustments'.

This way you can remove just enough material by keep trying the nut on the thread.

I would be afraid of removing too much with a die.

Patience is required.

[DaveKing]

If you have a friend with a screwcutting lathe I would give it to them to re-cut to just above size and finish with very fine cuts and test between each cut. They can pick up the original thread near the shoulder and work from that. As the end of the thread is damaged I would be wary of using a die as you do not know where it will pick up the thread and could damage what is left of the original bit.

[will_curry]

If the clutch is already off it's not a major job to get the mainshaft out but you will
need a suitable length of tube which will pass through the bearing in the inner
case.

Remove the kickstart mechanism and push the mainshaft out with the tube.
The tube should prevent all the innards from moving so that the repaired
mainshaft should slide back in place as it pushes the tube out. This method
has worked for me many times but it is best not to turn anything in the 'box
so if you'll need to move the bike without the mainshaft take the back chain off first.