If you have a late 1950s, 1960s or 1970s AJS, Matchless or Norton motorcycle, there’s a good chance it’s fitted with an AMC gearbox. As you will see from my website, I have several such motorcycles.
My 1964/65 Matchless G80CS, which identifies as a 1967 bike, had a habit of leaking oil from the gearbox kickstart, gear change and gear selector shafts…despite the fact that I had replaced all the seals and fixed a few other issues with the gearbox.
AMC Gearbox Inspection Cover
The giveaway was a particularly warm spell which caused quite a bit of incontinence from the gearbox. I remembered the tiny breather hole in the inspection / filler cap… on checking, I found it was well and truly blocked with black gunge. I cleaned it out with a carburettor jet cleaning tool and all appears to be well.
Why did the blocked breather cause such a problem…after all, the engine wasn’t running. Quite simply, as the ambient temperature rose, so did the temperature of the gearbox and the oil and air contained within. The result of that was an increase in air pressure inside the gearbox, and if it can’t escape via the breather hole, it will do so via the various shaft bushes by pushing past the seals. The temperature goes down overnight and air can be sucked back in…ready to be pushed out with more oil when the temperature rises again.
So…check your breather holes!
One other thing which can cause oil leaks to recur in just one season after replacing the various seals, is rough shafts. If your gearbox has been neglected at any time, there may be some corrosion on the kickstart and gear change shafts where the seals are fitted. Quite simply, the rough surface wears away the seal material prematurely. So, make sure your shafts are smooth and shiny before replacing your seals.