AMC Gearbox Conundrum

One of the final ‘issues’ the previous owner had mentioned, was that he’d sometimes had trouble engaging first gear, and that he would set off in second gear and first gear would thereafter not be a problem. I had asked when I inspected the bike, if there was enough oil in the gearbox and which oil he was using. He showed me a bottle of Silkolene Gear Oil Medium and told me it had been topped up when he bought the bike a few months earlier.

I had a good look around the various forums on the internet and found no definitive answer to the first gear issue…except that the late Mick Hemmings thought it was down to worn layshaft bushes. When trying to engage first gear, it sounded like the dogs were not making it into their slots. There was no gear grinding and the selection process was fine thereafter…until the bike was again stationary and in neutral. It didn’t jump out of gear either. In case you read these pages out of sequence, I thought it was the clutch dragging too, so I did that first…and I’m glad I did, as detailed on my previous page.

Bearing in mind the other things the previous owner had ‘not been aware of’, I drained the oil from the gearbox in preparation for an ‘in frame rebuild’; or at least I hoped that would be the case. The oil that drained from the gearbox was quite brown, not the golden colour of the Silkolene. What’s more, I reckon little more than 150ml made its escape from the box…there’s supposed to be 568ml in there, and despite the cold, I was pretty sure all the oil had made an appearance. So, the previous owner had never checked or topped up the gearbox oil, despite his assurances to the contrary. There had been no leaks.

I ordered a rebuild kit from Andover Norton and started stripping the box in readiness. Everything inside was in great condition, apart from three worn bushes; two were easy to replace and one needed pressing out and the new one pressing in. The latter was the bush on the layshaft first gear pinion, which also has the kickstart ratchet. I replaced the oil seals in the outer cover, the mainshaft bearing in the inner cover (as it did feel a little grumpy), upgraded the layshaft bearing to a roller bearing (an absolute must), replaced the cam plate plunger return spring as it was on the point of failing, cleaned the cover mating surfaces then used Wellseal and new gaskets. I also replaced the gear lever return spring after making sure it was bent correctly. Oh, and I put a full pint of oil in…that is what the 1966 ‘MA’ AMC gearbox holds, not 3/4 pint and definitely not 150ml!

Apparently, the original layshaft ball bearing fitted in the gearbox body, can’t really withstand the axial loads in the gearbox for too long. When it fails, one of the first symptoms, or so I’ve read, is that the kickstart starts having a mind of its own and tapping you on the back of the leg! If the bearing fails totally, the gearbox can lock up and there’s nothing you can do…not good if you’re cornering at road speeds. The upgrade is a roller bearing.

I’m not going to go through the stripdown and rebuild process in detail, as there are many great articles describing how to do so in a step-by-step manner with photographs; links are at the bottom of this page.

What I will tell you though, is what I learned from the process:

• Previous owners tell lies.
• The correct oil to use these days is indeed something such as Silkolene Gear Oil Medium.
• The correct amount is ONE IMPERIAL (UK) PINT (568ml if you insist on metric) which comes up to just under the fill level plug. Some sources insisted it was only 3/4 of a pint which is wrong for this G12 ‘MA’ AMC gearbox. By the way, you can look through the inspection and fill hole to see the oil level…no need to undo the fill level plug and make a mess everywhere.
• In my opinion, the worn first gear layshaft bush was the probable cause of the problem I had.
• Don’t try using a suitably sized socket to press out the old bush; the end of the bush will bell out and make it impossible to press out. I used a proper home made press tool from the other side which worked perfectly. The reason is that the end of your sockets are unlikely to be dead flat, which they need to be. The ‘wise man’ who suggested I try a socket, should have known this…and I should have realised.
• It’s worth doing this yourself as you’ll learn how the box works. Take your time and make sure everything is right.
• The Japanese bearing for the mainshaft fitted to the inner cover, as supplied by Andover Norton, is one thou too big. The English Hoffmann one I then bought from RGM Norton was perfect.
• Make absolutely sure you line up the clutch actuator precisely, to avoid a heavy clutch.
• Previous owners tell lies…did I mention that before?

The gearbox felt much more positive after the rebuild, and of course, it selects first gear from neutral when the bike is stationary with the engine running.

In conjunction with my period Haynes Norton Manual, which covers the AMC gearbox, I used articles from the vast archive at Jampot DK – which you can access free HERE.

There is another great source of free information at OldBritts HERE, but their archive is doomed to expire when their hosting money runs out; the owners of OldBritts have retired. I have made a copy of the website though, just in case.

I’ve taken two of their web pages and made them into PDFs which you can download and print:

• Dismantling and inspecting the AMC Gearbox and
• Assembling the AMC gearbox