Say you had a thread pull out of your block.

There are two options really, new thread insert (Time-sert or Helicoil) or over size head bolts/studs.

What effects would it have if you used M10 instead of the standard M8?

 

I wouldnt want to use a different size bolt unless you did it on ever single bolt as otherwise you are going to get uneven clamping force.

Just helicoil it and use the proper bolts (or better still studs)

 

Of course, i would never dream of just over sizing just one of the bolts.

Is there any reasons why you should not increase the dia of all the head bolts/studs?
Afterall a lot of people use 12.9 high tensile bolts so i presume increasing the dia will have no adverse effects

 

I guess the only problem would be if your bigger holes are getting closer to waterways or whatever - it'll be weaker then

 

The problems are:

getting the correct claming force on the gasket and not crushing the fire rings by doing it up too tight (but thats just a case of using the correct torque settings on the new bolts, but dont assume they will be the same as the old ones!)

Weakening the block by taking more material out of it, but TBH this is the same for a helicoil anyway as you still have to drill it just the same.

 

PS

dont forget you need to drill the head too on the bigger bolts, and that could crack it!

I honestly cant see ANY advantage in using the bigger bolts, why do yo want to?