Upping manifold stud size

[ADrewzki]

I've had to retap a few of the manifold studs on my head just due to age and a few of them are slightly crooked. It's exacerbated by the fact that I have a 1/2" spacer plate for the manifold. I'd like to drill and retap everything and go with the biggest size possible in terms of a stud or bolt. How big can I drill that hole out to?

My biggest worry is that I'll hit an oil passage or something in the head which means I'll have to get a new one. Current size is 8mm I believe with the far left/right studs being 10mm. Maybe go up to 10mm on all??

 

[Cs4g63]

I know you can safely drill and tap all of those for a heli-coil or fix a thread, but I dont remember how big that makes them. Im at work right now so I dont have the helicoil kit to tell how big the drill bit is, but that should give you an idea of what you can do.
Hope this helps.

 

[ADrewzki]

I know you can drill it out and put the stock 8mm size helicoil in there. I wanted to drill it out bigger and put a helicoil for 10mm in there. I was worried I'd end up drilling into an oil passage or something.