Cheers for suggestions, I will try first the hex that is slightly larger and hit it so it makes the edges flat again. If it dosent work (or if I can't do it) then I will try the 'creating a flat head screw' I initially didn't want to harm the countersink, but I will getting an engine rebuild shortly so will have everything replaced. cheers

Do you know what the dimensions are ? Or what other bolt would fit? Cheers again.- James.