G12 oil leaks

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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G12 oil leaks

Postby neil cairns » Tue Jul 09, 2019 7:30 am

My '59 G12 has leaked from the big chrome filter nut for some time. Its OK when tightened up but loosens off (possibly with vibration) just enough for drips under the engine. I've tried many seals, etc, but have finally settled on two fibre washers, PTFE tape and wire-locking the damed thing. This after on Saturday the engine dumping nearly all its oil after a rubber seal I'd fitted blew out (not enough rebate). It has a releif valve inside it so drilling for wire-locking had to be done carefully (ex-RAF aircraft engine fitter). It also leaked from the oil feed through the head gaskets, which I modified to take tiny BMC valve stem seals by filing out the gasket hole a bit. This took some time to find as the leak appeared as if it was from the crankcase between the cylinders, I did rebuild an engine for a mate of 1962 and this had copper head gaskets and oil seals, so I copied it. The dynamo simply would not seal and I ended up with two cork gaskets and a Gee clamp holding the dynamo hard against them as I clamped it up. The bike is called "Torry Canyon" by the way.

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Re: G12 oil leaks

Postby dave16mct » Tue Jul 09, 2019 10:50 am

The valve in the filter cap is a non-return valve not a relief valve. It's to stop oil from draining down from the top end when the engine has stopped. What oil are you using? I've used 20/50 for the last 40 years on my twins with no problems but my on latest twin I just can't stop some leaks. I'm even considering straight 50 oil as Rob Swift recommends. Fed up having my left boot covered in oil!
Dave.

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Re: G12 oil leaks

Postby neil cairns » Fri Jul 12, 2019 6:04 am

I'm using 20/50 with a big dollop of Wynns. I read somewhere that the oil pressure with cold oil can get up to over 200psi.

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