Ta very muchly Beezageeza, appreciate that tip a lot, and it makes 110% sense. In fact, I have checked that very thing and it was OK, in fact quite a fierce spring and plunger I thought - perhaps too far screwed in (plenty of signs of a nutter on the end of the squared shank with a worn out pair of mole grips). Maybe someone had already tried to sort out the same symptoms via the wrong route . . .
Anyway, went ahead after that and other checks on the bits we can all get at . . . but no dice. So took the whole thing down this pm, and found what I as a BSA novice thought were interesting things . . (after I'd washed the outside enough to see the allen screw (why is mine metric-headed?) and figure out how the darn selector fork rod comes out of the casing, that is!)
. . . to wit - mainshaft pinion as wot locks the sleeve gear (top gear) into direct drive was having the trouble of a drunken Lothario achieving any degree of satisfactory penetration . . . unnatural practices had led to unnatural wear on the extremities of the engagement dogs. Nothing much else looking too knackered, including the mainshaft selector fork . . . except for the fact - and here I have no idea what the parts should look like not having as much as a simple exploded view of a BSA gearbox to hand - that the sleeve gear bushing was weird. Are there meant to be two bronze bushes, or one; are there meant to be two of equal length or a long and a short; or just one long one? Whatever, the combined length of the 2 bushes that were in this sleeve was about 1/4 inch longer than the sleeve itself . . . so some loony had hammered the end of the bush that was sticking out on the g'box sprocket side in a bid to try to make it flush when doing up the final drive sprocket . . . net result, inner of the 2 bushes (if there are meant to be 2) on this sleeve was protruding well into the recess on the 'dog' side of the sleeve gear pinion and preventing more than about 30% engagement, in depth terms, of the 2 pinions. Ergo it had lasted a while, then the dogs had fretted enough to make a poor connection worse.
Remedy, if I can get the bits, is a decent sleeve gear and adjacent mainshaft pinion, plus new bronze sleeve gear bushes if they can be had. Or even a full set of main- and lay-shaft with pinions in decent nick.
If the bits can't be got - and I understand from this pm on the web from deepest France that pinions are tough to get - then I'll just turn up new bush(es) and see if the thing will work for a bit while pondering where to go next to replace the pinions. Haven't a clue whether, ref other comments here about washers and spacers etc, I'm also missing any bits completely - but the only washer or spacer in the entire box inboard of the inner right hand end case was one between the outermost layshaft pinion and the first of the splined ones operated by the layshaft selector fork . . . no idea how good or bad that sounds to them as knows about these things.
The only other sleeve gear I've had apart in recent times was on a Burman - in which there are 2 (more substantial) bushes, spaced apart and a decent press fit in the sleeve. If anyone has the dimensions for cutting new bush(es) to ensure a good fit in the sleeve that would be great . . .otherwise it'll be measure and make and fit and refit and ream on the inside till it all goes together just so . . . 'Course, if anyone has a reasonable second hand box they don't want from which I could make a decent hybrid or indeed just swap out the guts, my gold awaits . . .
The good news was that many of the other parts weren't at all bad, bit like the curate's egg. Plus, natch, I've enjoyed myself and learned a lot. Way to go!
Groily
