Hi All
Have now removed my Burman Box and found the following :-
The output bearing housing had been modified to take a sleeve so that a "standard " bearing could be used and the flange on the inside was broken off, but with enough left to stop the sleeve going right through. This I already knew, but the sleeve had become loose in the gearbox casing and the bearing had since worked loose in the sleeve, allowing the output gear to move in if it wanted to. I'm surprised there were not nasty noises as the output gear touched the selector fork casting or a gear on the layshaft !!
This could however have resulted in better meshing of 4th gear, so the marks (about 4mm) of the meshing of the gears (see attached photo) may be false ones.
Even though I have had the bike since 1956, I did not do this mod - so the box must have had a serious problem before this.
I have tried my gears in the "new" casing and get nearly 4mm of meshing into 4th gear with all tolerences in the worst direction - but get very much the same figures with Vic Youle's old parts !!
I had about 0.6 mm of backlash on my selector shaft which I have got to nearly zero by putting a shim behind the steel bearing which the rollers go into.
So far therefore I can not account for the small meshing of Vic and tatty500's gearboxes unless two things have happened.
1. There is a lot of backlash in their selector shafts - which with their measurements does not seem to be the case.
2. Or their output gear has moved outwards for some reason.
I shall put a 1mm shim behind the output gear and have checked layshaft end float (about 10 thou) .
With new bearings etc I am fairly confident my gearbox will be OK, but some puzzles still remain on the cause of Vic and Tatty's problems as far as I am concerned.
Will come back onto the forum if I find anything else.
Matty