1966 G12 with MA version AMC Gearbox difficult to engage first gear

[nomisimon]

I've created this new topic to keep the subject free from another one.

My 1966 G12 is almost impossible to get into first gear when stationary with the engine running. It will go into second just fine and from then on, up and down the box whilst on the move, although it feels a little 'squidgy' if that makes sense. When trying to engage first with the engine running when stationary, it's as if the lever comes up against a stop. If you lift a little harder there is a soft clonking noise. As I said, it goes straight into second and works OK thereafter. Moving the bike backwards and forwards a little doesn't ease the issue.

I thought it was the clutch too, but stripped the primary drive and found the clutch centre nut not even finger tight and the clutch flopping about somewhat. The clutch shock absorbers had been melted, the three cover screws were finger tight and the chaincase had obviously been filling with oil; the latter has been fixed. I've replaced the shock absorbers, cleaned and dried all the plates and other components, before reassembling correctly. I thought all would be well, but sadly not much difference.

There's a great deal on various forums about this issue with AMC boxes and it seems fairly common. Having rebuilt my B52 box on my G3LS, I'm intending to delve inside this one. The late Mick Hemmings allegedly blamed worn layshaft bushes for the problem.

I wonder if anyone here has the definitive solution? Thanks.

[dave16mct]

It sounds like the quadrant has been fitted on the camplate one tooth out. Have a look at this it explains everything. you'll need it to rebuild the box anyway. https://www.oldbritts.com/gearbox_a.html
Dave

[nomisimon]

Thanks Dave...there are a number of useful articles attached to that one; I'll save them all. The gears are all selectable when the bike is stationary (engine stopped) or moving (engine running) so I'm not sure what's wrong just yet. Having just rebuilt the primary drive, I'm hoping that whatever needs doing, can be done with the box in situ. If anything is even suspect though, the box will come out.

[mdt-son]

> I wonder if anyone here has the definitive solution?

There are no definitive solutions, as every bike wears in different ways due to riding style, maintenance related issues, or (seldomly) part failure(s). I'd disassemble the gearbox and inspect shafts for straightness, then inspect the bearings and bushes for wear. Selector forks may be bent.

- Knut

[nomisimon]

Hi again Knut. Yes, that was my conclusion too. I've almost finished the gearbox, having replaced some worn bushes, especially the one on the first layshaft gear, and upgraded the rear layshaft bearing. The index plunger spring was also at failure point, but not causing a problem, and I've replaced the seals etc. Otherwise the gearbox looks very good. I think the issue was that the dogs which engage first gear could not find their holes properly; a worn bush could cause that. I've just received another outer mainshaft bearing, as the replacement Andover sent me was a thou oversize. Once that's done I can check the layshaft end float, shim where needed and finish reassembling the box. Cheers, Simon.

[nomisimon]

The gearbox is now back together and road tested. It seems much better all round and goes into first gear fine now. I suspect the main culprit was the worn bush on the first gear on the layshaft, for those that haven't been inside, it's the one with the kickstart ratchet built in.