My mid 60s Matchless G80CS turned out to have a leaky cylinder head to cylinder joint; I could see small oil bubbles at the joint and exhaust coming out from the right hand side of the joint.
Annealing my copper head gasket
These engines have a compression ratio of 8.7:1 and no cylinder head gasket; the head and cylinder joint is lapped using grinding paste. There are some other issues to resolve at a later date, so I decided to fit a copper head gasket, designed for the road-going Matchless G80 or AJS Model 18.
Fitting this gasket reduces the compression ratio slightly, to around 8.2:1. I’m reliably informed I probably won’t notice the difference…challenge accepted 😉
Note that when fitting a gasket, you will be raising the cylinder head by about 36 thou. This means you will have to readjust the pushrod clearances, and use slightly thicker rubber o-ring seals on the end of the pushrod tunnels.
I bought my head gasket, along with new o-rings, rocker box gasket and inspection cover seal, from Steve Surbey at AMC Classic Spares.
Before you can fit a new copper head gasket, you need to anneal it. This involves heating it up to a cherry red colour, leaving it like that for a few minutes, and then quenching in cold water. The resulting copper will then be very soft and pliable. Most people suggest using welding torches or the like to heat the copper gasket. If you still have a gas hob, just put the cleaned gasket over the most suitable burner and heat it up. I didn’t think it would get hot enough but…every day’s a school day.
Cylinder head refitted
Once quenched in the sink, the copper gasket is ready to fit.
DO NOT overtighten the cylinder head sleeve nuts…the correct torque wrench setting is 40ft lb or 54Nm. There should be a spacer under the sleeve nut, 3/4″ OD and 1/8″ thick…the ID is of course 1/2″.
This is one of those jobs best undertaken, when the wife is out 😉