Greetings all. Ok,so ive recently completely rebuilt a 1942 ish WDM20. It starts up fine and runs well for afew minutes before cutting out. Removal of the plug at this stage reveals a sooty plug with almost no gap.Im using a NGK B6HS,and from memory the inlet valve is standard,as is cam. My question is, is it possible for the inlet valve to hit the spark plug electrode,even when correct lenght plug is being used? Clearences,cams and plugs have been checked & rechecked!Over to you,M20 boffins! regards Hamish NZ

email (option): fuzzyempire@hotmail.com

Hi Hamish..Well, you have the right spark plug and under normal circumstances there would be no chance of contact between the plug and valve head.
Points to consider are..has the head been excessively skimmed? This would 'drop' the spark plug. Has the spark plug seat been re machined?..check to see how far the plug is protruding from the head.
I'm guessing the two parts are only just contacting..However, if you have a 'witness' mark on the top of the valve I would suggest removing the valve to check that it is still straight...Ian

email (option): ian@wright52.plus.com

A chap I knew had the same problem on an old Austin and he simply fitted another washer onto the spark plug, thus shortening the thread. Not an ideal solution as you'd obviously be better off finding the fault instead of bodging it, but I just thought I'd mention it.

If it is hitting due to an over skimmed head a thicker solid copper head gasket might fix it.

Cheers Pat

Thanks for the tips guys. Ive been using that double washer bodge up til now . Time for further investigation.

email (option): fuzzyempire@hotmail.com