GAS­KETS AGAIN

Real Classic - - Letters -

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

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