G12 chaincase breather/alternator holes

Hi - I have my friends Matchless G12 1961. The chaincase leaks quite badly following a ride but when left unridden doesn't seem to do so. I usually work on my BSA A10 so need to get my head around the Matchless breather. I think that the crankcase might breathe into the chaincase and there is then a breather hole from the chaincase to atmosphere (just above the hole where the alternator wires exit). (I seem unable to upload a photo - don't know why but I only get the option to upload from a URL) Upon inspection I notice that someone has inserted a plugin this upper hole. This is a solid plug so is not venting to atmosphere. If I am right then this 'plug' needs to be removed to allow venting. I see no reason for the maker of the chaincase to have made this hole only for it to be plugged! Can anyone confirm if I am right or barking up the wrong tree? I can send a pic directly to anyone able to help if necessary.

Cheers, Chris

Gotta get the VIP membership to upload images easily or have a website where images are stored and point to the image with the URL.

The primary cover setup is difficult to seal on the best of days. Only time mine didn't leak when I ran a wet clutch on a P11 (same primary case) was when it was parked after I cleaned off the bottom of the primary.

The crankcase should have a timed breather driven by the end of the cam just above the primary cover at the front drive side of the crankcase. I'm not sure where it vents to on a G12, but it should not go into the primary case. On the P11 it goes up to a froth tower on top of the oil tank.

Schwany said:

The crankcase should have a timed breather driven by the end of the cam just above the primary cover at the front drive side of the crankcase. I'm not sure where it vents to on a G12, but it should not go into the primary case.

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Matchless G12 engines are different to the Norton twin engines. The c/case vents through the crankshaft and a flapper valve fitted inside the rotor nut (alternator version). Dynamo versions had a similar arrangement.
The flapper valve's function is similar to aftermarket bolt-on reed-type valves available for Norton engines. How well the flapper valve works I cannot say.

Spottycat said:

Hi - I have my friends Matchless G12 1961. The chaincase leaks quite badly following a ride but when left unridden doesn't seem to do so. I usually work on my BSA A10 so need to get my head around the Matchless breather. I think that the crankcase might breathe into the chaincase and there is then a breather hole from the chaincase to atmosphere (just above the hole where the alternator wires exit).

Click to expand...

There should be two holes in the chaincase behind the aperture for the crankcase: One for the stud bolt supporting the chaincase by the crankcase, and a hole located under the chain deflector for the alternator leads.

A separate breathing hole isn't necessary, as the g/b mainshaft passing through the sliding disc allows for a liberal amount of breathing.

Please verify the mating faces of your chaincase are flat. There should be a gasket present. Obviously you are experiencing an ingress of oil from the engine. This is probably due to a worn oil pump and too much crankcase pressure caused by worn piston rings.

- Knut

mdt-son said:

Matchless G12 engines are different to the Norton twin engines. The c/case vents through the crankshaft and a flapper valve fitted inside the rotor nut (alternator version). Dynamo versions had a similar arrangement.
The flapper valve's function is similar to aftermarket bolt-on reed-type valves available for Norton engines. How well the flapper valve works I cannot say.

- Knut

Click to expand...

Alrighty

I messed up and only looked at Norton G12 images returned by Google, because I am not familiar with anything other than that style Matchless primary, and the few images showing the drive side did have the timed breather hose fitting off the crankcase at the end of the cam. My mistake.