G80 CS barrel crankcase gasket

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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G80 CS barrel crankcase gasket

Postby martinco5 » Thu Dec 28, 2017 3:37 pm

Hello to everyone

I have a 1961 G80 CS engine in several pieces, trying to find lost compression. With the cylinder barrel now removed I find there is no gasket between the barrel and crankcase. I have read that there is no gasket between the barrel and cylinder head but I have not seen a similar comment in any of the manuals about the barrel and crankcase.

So should there be a gasket there or not?

I have removed the top piston ring and pushed it down the barrel to check the gap. the manual states the gap should be .010 - .015 in, but the largest gap I have found in various parts of the cylinder bore is .004 in. I have made sure to keep the ring square to the bore when making these measurements.

So, is this the latest thinking that ring gaps should be tiny or should I get the files out, maybe splash out on a new piston.

I hope everyone enjoyed Christmas and I wish you all a Happy New Year.

Martin

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Re: G80 CS barrel crankcase gasket

Postby Mick D » Thu Dec 28, 2017 4:35 pm

Hi

The relevant parts manual indicates that you should have a paper gasket between the cylinder and crankcase, (part number 0222515), I'd fit one if it were my bike.

With regard to the ring gap - have you tried measuring the gap at several positions around the cylinder? Unless the ring and cylinder are new / re-bored they tend to wear ovally, orientating the ring in the cylinder in anything other than it's normal running position will give a false, (reduced), reading.

I take it you've looked at the valve seats and the tappet clearances were good.

Regards Mick

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Re: G80 CS barrel crankcase gasket

Postby martinco5 » Fri Dec 29, 2017 10:23 am

Thanks Mick

I’ve measured the ring gap at several points up and down the cylinder but I’ve not rotated the ring as you suggest.

Valve seats were not brilliant but they have lapped up to look right ie a clean grey matt finish. Tappet clearances not checked before dismantling engine, silly over site. Its a long time since I did any engine work, I’m obviously out of practice!

Regards

Martin

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Posts: 5
Joined: Tue Jun 25, 2013 1:27 am
Location: FRANCE

Re: G80 CS barrel crankcase gasket

Postby martinco5 » Fri Dec 29, 2017 7:21 pm

I’ve spent the afternoon measuring things as follows: -

Cylinder bore diameter 85.4 m.m., should be 86.0 mm for a G80 CS imported from the States.
Piston diameter is 85.0 mm
Piston/bore clearance is .021”.
Top ring depth 3.0 mm
Piston ring groove depth (top ring) 4.1 - 4.4 mm at various points around the circumference
Top ring width 1.8mm
Lower ring depth 3.3 mm
Piston ring groove depth (lower ring) 4.2 - 4.4mm

Piston ring gap .002 - .003” when measured at various points in the cylinder and also when ring is rotated as suggested by Mick.

No sign of a lip at the top of the bore.

Regards

Martin

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