I tend to use big 600 sizes and find that the current head gaskets do vary a bit. Holes for the 5/16″ studs are punched 10mm, but even then sometimes I have to file them out a bit.
There is usually so much clearence that you must drop the pistons in backwards without rings to locate the gasket before you tighten any nuts.
On one occasion, I had to make a dummy piston with a lead on it to swage the copper back a little so the piston would clear. For the engineering minded, on my engines where I have to accurately position the contoured high compression head profiles, I counterbore the two extreme end holes and fit 10mm OD 8mm bore ring dowels, so I can be sure of position. The variation in the gaskets would indicate a manual operation rather than a CNC production method. Nobody promised it would be easy! Kindest Regards Roger
