GASKETS AGAIN

My 1959 Matchless G12 was rebuilt by me three years ago and since then has developed many odd oil leaks that I am slowly sorting out. It starred in RC some years ago when its previous owner failed to turn on the oil tap in the main feed line and I rebuilt its engine. With these oil leaks the picture gets clearer as you cure the obvious ones: the big leaks swamp the little ones!

One leak I found but failed to cure completely ran down behind and between the cylinders. This worried me, because there’s an oil feed channel between the two crankcase halves and to cure that would mean stripping everything down again. The oil leak I had ‘fixed’ was from the composite head gasket on one cylinder head. The oil feed goes up through a hole in the gasket very near the outer edge. At the time a new gasket and some Hylomar seemed to have cured that leak, but it re-appeared.

There appeared to be not enough clamping pressure on the gasket’s large surface area to grip around the oil feed hole. So I bought another two head gaskets and, this time, carefully filed out the oil feed holes in both to be able to fit a BMC A-series engine’s valve guide seals. These are very small, and my aim was to get a secure sealing around the oil feed area. It worked!

Neil Cairns, member 2501

‘Heads Up’ in the letters of RC171 contained some fascinating historical detail. However, I feel I must take issue with one small point. Your correspondent states that reducing the area of a gasket increases the clamping force applied to it, and goes on to say that ‘there is no need to increase the tightening torque on the cylinder head nuts’.

There is no increase in clamping force. What is happening is that, since the area of the gasket is reduced, then the clamping pressure (force per unit area) is increased. I’m sorry if this seems a bit picky, but since your magazine and contributors maintain such a high standard of engineering, I thought it worth pointing out.

Peter Cushnaghan, member 7906