G80 oil pressure relief valve

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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G80 oil pressure relief valve

Postby AndyO » Wed Dec 23, 2020 10:26 am

After starting and running smoke free for 5 minutes my 54 G80 starts to smoke. It’s had a rebore, new piston, rings, valves, guides etc. Someone suggested I check the oil pressure relief valve and/or crankcase breather.
I’m a complete newbie to the marque. Where are these located and they easy to check? Thanks everyone

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Re: G80 oil pressure relief valve

Postby SPRIDDLER » Wed Dec 23, 2020 11:00 am

There isn't an oil pressure relief valve, just the crankcase breather.

It's just a free steel disc (Pt. no. 000835) within the nut (000836). For an illustration see page 9 in the Spares list here:

http://archives.jampot.dk/book/Spares_l ... ingles.pdf

It's here, behind the inner primary chaincase.

Breather - crankcase marked.jpg


Can you describe the seriousness of the smoking? A rebored engine will smoke a bit until the rings bed in.
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Last edited by SPRIDDLER on Wed Dec 23, 2020 11:07 am, edited 3 times in total.
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Re: G80 oil pressure relief valve

Postby Greybeard » Wed Dec 23, 2020 11:05 am

How many miles since the rebore/piston? I'd give it a few hundred to settle down and for the rings to bed in before getting too worried. You could check the oil feed screw for the inlet valve in case thats been set too generously. All the details are in The Manual or the tech articles section.

Steve

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Re: G80 oil pressure relief valve

Postby ajscomboman » Wed Dec 23, 2020 11:27 am

To check if the valve is working you only have to run the bike and either listen or stick your finger near the end of the pipe and feel the air pulses, if there are none to be felt and no air puff can be heard then it's likely the disc is stuck. That will cause plenty of smoke out the tail pipe.

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Re: G80 oil pressure relief valve

Postby g80csp11 » Wed Dec 23, 2020 12:09 pm

the other way of checking is to put a piece of rubber pipe on the copper valve tail Spriddler shows and blow / suck on pipe . blowing will close the disc and seal wher as sucking will open disc and give you a mouth full of crankcase fumes . PS dont have the engine running when you do this check !

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Re: G80 oil pressure relief valve

Postby Pharisee » Wed Dec 23, 2020 1:28 pm

Have you checked that oil is returning to the oil tank? If it's not for some reason, oil will build up in the crankcase and give you some serious smoke from the exhaust.

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