Gearbox removal and dismantle

I usually slacken the nut while it's still in the frame. If you don't then the easiest method is a short length of old chain clamped in a vice afterwards but it's more fiddly.

The Commando box is a nice straighforward thing with little in the way of shims to lose or fiddly setting-up. If you follow a clear set of instructions, it will be fine !

I have used both methods but try to slacken the nut using the chain and rear brake to clamp the sprocket as you describe. You may find it best to remove the mainshaft first if using a box spanner so the tommy bar can be placed close to the nut.

I would be surprised if the rear brake was up to the job unless the nut was already loose. (They often are) give it a go, but you have a mechanical advantage over the rear brake just by the ratio of gearbox sprocket to rear wheel sprocket.

It may be that your sleeve gear bush has moved. If you use the normal two bushes there is a gap between them which means they can move. Quite often they move out and get chewed away by the clutch retaining circlip but they could move inwards too. I use three bushes or I believe the Mk 3 bushes are longer so you could use them. I also put a washer on the main shaft between the bush and the clutch circlip so that if the bush does move it doesn't go far or get chewed up. As in the picture.

It is possible to rebuild the gearbox with the main case still in the bike. So you could open up the box up and remove some or all of the internals before deciding if you would be better to remove the gearbox. An of course it is the ideal time to look at the layshaft bearing which is a well known weakness of the gearbox.

I agree with David Evans, if the nut is really tight the chain gearing over-rides the rear brakes power to hold the wheel stationary. Sometimes the nut is very loose, sometimes they are nearly welded on and DO NOT forget they have a LEFT HAND thread....the only one on the box.....best way if really stiff is to get a metre long steel bar and use a short length of old chain around the sprocket the chain can be fixed with two small bolts through the bar if you knock out the end links....If really tight a blow torch might help by heating the nut. you can get at it easily enough.

Les

Sleeve gear nut chamfers facing in? It sits nicely on the sprocket that way and gives a little more material for the socket to get a hold. Its probably not vital though.