Paul....Compared to earlier BSA designs, this four speed box is almost too simple, and the straightforward service sheets 310 and 311 detail the stripping and reassembly. The external, almost hidden grub screw on the selector fork shaft is sometimes troublesome to undo, but with this screw removed and the shaft pushed into the box and slid out, the whole of the internals (except the final drive sleeve, bearing, sprocket and oilseal) can be removed with a bit of gynaecological manipulation.
Once apart the condition can be assessed and the job entrusted to the professionals if necessary. Both the earlier Plunger box and S/A type can be stripped almost to the basic shell plus the final drive as outlined above with the box still in the frame.
If refurbishing a box, leave that grubscrew off until the inner case and rod are finally in position. With the grubscrew tightened against the rod, knocking the inner case back on with the selector shaft misaligned or tight in its hole will break off a piece of the shell casting.
Faults...Look for worn ball races, worn and scored shafts and plain bushes. Layshaft blind bush always suffers. Gear teeth and drive dogs do wear, but most boxes can be brought back from the dead, the bushes, bearings etc are widely available. Camplates wear in the selector tracks but are fairly cheap to replace.
Plenty to read in previous posts in the gearbox section of the Forum.
Swarfy.
