When I did the work on my 1966 Matchless G12 gearbox and clutch, I drew some of the reassembly information from similar Norton chaincases. This was wrong. The cast aluminium chaincases AMC used in the mid 1960s were used on a number of Norton, AJS and Matchless motorcycles. They weren’t all the same I’ve discovered.

G12 inner chaincase

Once I’d reassembled the chaincase and refilled it with oil to the correct level, there were no leaks. However, every time I rode it, there were frustrating leaks under the primary chaincase, which stopped after the bike had been standing a while.

There are two holes in the middle of the back part of the chaincase. The top one has an aluminium moulding over the top on both sides of the chaincase to prevent oil splashing in the area of the top hole; to some degree, the lower hole is partially protected by the same moulding. I couldn’t find any information on the chaincase at the time, but the alternator cable had originally been fed through the upper hole, and this was also shown in images of similar chaincases. I put it back like this, assuming it was correct and there would be minimal oil leaking out…sadly, not the case.

Time to ‘bite the bullet’ and fix it for the 2023 season.

Closeup of the two holes

I stripped the chaincase cover off again, as per my previous method and inspected the holes. The top one has a hole right through, about three quarters of an inch including the protruding boss. The lower hole has a recess in the back of the back part of the chaincase. and a smaller hole. Further investigation was necessary.

The Matchless G12 engine, has a ‘flap’ on the end of the crankshaft, allowing the engine to breath…into the primary chaincase. Anyone who has a bike that wet sumps, will know that once the oil in the sump gets high enough, it leaks out of this breather and fills the primary chaincase…before leaking out of its breather on to the ground; mine has done this, but now has an oil tap linked to the ignition.

Scouring various articles and applying a bit of logic, confirmed that the top hole on my chaincase is actually a breather hole. It’s just under half an inch in diameter and open to the atmosphere…so water and road dirt can actually get back in.

Grommet installed

The bottom hole was clearly for the alternator cable, but surely it needed some form of grommet to stop oil escaping? The Andover Norton website had part ‘06.0903 – Grommet – alternator‘; there is a second part number 04.1065 mentioned too. I ordered a couple as they were 60p each…they fit the bottom hole perfectly and the grommet hole is the correct size for the alternator cable. Note that the protruding part of the grommet is on the inside of the chaincase; I challenge anyone to fit it the other way round, without removing the chaincase first.

I can’t find the above information easily anywhere on the internet, so hopefully this helps. This information is for the last Matchless G12 / AJS Model 31 650cc twin motorcycles, which use the cast aluminium chaincase without the external centre stud. It may apply to some of the later AMC singles too. Norton engined bikes are not the same.

I wasn’t happy leaving the chaincase breather, the top hole remember, open to the atmosphere. Whilst pondering the issue, I asked the advice of a very knowledgeable gentleman who looks after AMC Hybrids in the Norton Owners Club. He explained exactly why the breather is as it is, and as a result, I’ve decided to leave it as AMC designed it and reassembled the chaincase. The alternator cable goes through the lower hole, via its grommet.

I’m a member of the Norton Owners Club and have found them very helpful indeed.

Today, the 15th May 2023, I took the bike for a short run; there was no evidence of incontinence.