Bringing M8 bolts down to M6?

[luadraman]

All the bolts on my crankcase are M6's except for the ones going through the oil filter cover, these were once M6's too but thread was stripped and they were tapped up to M8 (only option on the road at the time). The metal around one of the bolt holes through the oil filter cap has cracked open because the increased bolt size means bigger hole for the bolt means less metal around it means less strength.
There seems to be a bit more metal around the holes going through the crank case cover and crank case itself so I'm not worried about them (yet!) but if I replace the oil filter cover and drill it bigger again it's probably going to happen again.
I was looking into helicoils and time serts but they seem too thin to take hold in the enlarged holes.
Anyone have any bright ideas?
I had thought of a time sert within a time sert or something like that but I have no clue about these things so I'm presuming that'd be a bad idea.
Thanks!