Gearbox oil leak

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My vintage car (1925) has an bad oil leakage due to the fact that the drain sump bolt has no washer or gasket. The drain plug is a solid brass threaded into an alloy sump, both threads are fine and present no problems there. My question is that I have just tried a blue instant gasket which did'nt stop the leak, any suggestions as to the best material to use for a washer/gasket? Thanks.

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Thanks Steve, so simply a flat copper washer then thats a snug fit?

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Thanks guys, Dave all I can say is that the brass sump plug is threaded on most of the shank except the area immeadiately below the bolt head. So an unthreded length of about 1/4 inch or so. The base of the nut is flat and the gearbox mounting has an flat ringed area where the bolt tightens up to. Sorry its difficult to explain. Oil loss is about a pint per month maybe slightly less and leaks hot or cold.

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All sorted now, found a slight bit of damage so was'nt quite flat allowing oil thru.