AMC gearbox problems

Depending on where the leak is you should be able to work on the gearbox while it is still in the frame, if a major overhaul is needed you can dismantle the primary side so the mainshaft can be removed but it sounds like you would not have to do that unless the leak is from the gearbox sprocket seal. I would track down the source of the leak before you open up the box as you will remove the evidence then.

My AMC box also kept jumping out of 1st gear, and in the end, the answer was to shim the layshaft, as it had too much end-float. Commando rear isolastic shims are perfect....https://www.oldbritts.com/gearbox_a.html see here.

My 1057 Dommie 99 jumps out of bottom gear frequently, and also the box loses oil. Can I remedy this with the box in the frame, and if not, can I remove the box without taking the engine out? Any help about what's going to need replacing would be most helpful. The box is AMC. Many thanks for any advice.

Well let say you can strip the AMC gearbox from the right hand side but you would need to remove the clutch and you need clutch tools to do this the right way, and jumping out of 1st gear is the camplate plunger spring getting weak

or the gearbox is out of timing they can go out of timing over the years and the selector spring also get weak and some time willl not select gear right, most gear box leaks are from the kickstart shaft oil seal, or poor sealing on the inner gasket, screwdrivers used as chisels to prise of the covers and leave gouge marks on the alloy mating surfaces, then you have a leak, the times I have seen its practics along with Chisles marks on Nuts and Bolts, if you have not got the right size tools is very easy to make a spanner from a peace of flat bar all you need is a good hack saw and a drill and basic engineering principles yours Anna J

I would find the leak location first, that will dictate which side to attack the gearbox from, on the clutch side the gearbox sprocket seal is the only source (unless the primary chaincase is leaking but the oil is reaching the gearbox exterior making it look like a gearbox leak so check the oil leaking is gearbox oil, if you use EP90 it smells awful so is easy to spot). On the timing side it could be the 2 cover joints, the kickstart shaft (an O ring on the Commando, was it used earlier). You will need a new selector spring, shims for 1st gear possibly, gaskets and the kickstart O ring if it was used on that box. Shimming alone may not work and you will need to examine the dogs and the holes they engage with to check for excessive wear. If they are then new gears may be needed too.

I agree that shimming alone may not be the answer. My box was like new inside, it was just an end-float issue, so shimming worked, but if you have noticeable wear, then parts will be needed.