A couple of times in January (temp-6C) my gearbox froze.usualy soon after starting out the gearbox would not shift from top,the lever would move but not shift gear.A bit of joggling with the toe would sort it,then all Ok.Today it did it at normal temperatures this required stopping ,engine and all,and rocking to get it to shift.I know BSA did a Maudes trophy that involved an M21 crossing London twice with the gearbox locked in top,but the hills round here prohibit that option. Any ideas?

Cheers Pat

Hi Pat,
I've never had this problem so don't have a definitive answer but I'd be looking for the easy fixes first. Take the side cover off first and check the operation of the ratchet and pawl and that the pawl springs are doing their job, then the adjustment of the control rod (this can be a tedious job. If all this seems in order then the box may need to be removed to check the action of your 3rdth gear selector dog, this can be done by removing the inspection plate at the back. Let us know how you get on.

Hi Pat..It is highly likely your box has been incorrectly assembled or set up. Either the shafts have been incorrectly shimmed on assembly or the setting of the dogs (using the adjustable tie rod) is incorrect...or both.
Each selector fork has a cam track machined into it and as the selector shaft is turned a peg pressed into the shaft moves along the cam track and the selector fork transmits the movement to the selector dogs.
3rd and 4th gears are on the mainshaft.. when the peg moves along the cam track in the selector fork between these two gears it has to negotiate a 'corner' in the cam track.
If the box is not correctly assembled or set up the peg jams against the track wall at the apex of the bend. This can make the change between 4th and 3rd stiff..or it can stop movement altogether.
Sometimes if the change is stiff it will improve as the peg wears the apex of the corner of the cam track.
This isn't really good and the selector fork and the slot it fits into in the selector dog can also wear badly and overheat...
Unfortunately the only way to check the operation and setting of the box is through the inspection plate and that can't be done effectively with the box in the bike.
You may get a result by adjusting the tie rod between the gearchange assembly and the selector shaft operating pinion, which is inside the outer cover, but this will be a 'blind' adjustment based entirely on chance.
It sounds like decision time as to whether you try this or take the box out for a more comprehensive inspection....Ian

email (option): ian@wright52.plus.com

email (option): viaconsu # planet dot nl

Sounds about right going buy the rest of the bike.There are tell tale signs of the mad bodgers inside the outer cover,heat applied to the adjuster link,no inner gasket etc. It will probably work out like last year,in bits April to November,but I never like riding in the summer,the heat and the flies.Back to the T3 again

Cheers Pat

Have you checked the obvious, like a stripped or loose spline on the gear lever?