Stripped threads help
 

One of my 300E's has a stripped 10mm x 1.0 coolant bleed screw on top of cylinder head, and I need to fix it.

Will a heli coil hold up to the pressure of the cooling system or should I just tap it to 12mm and use a new 12mm bolt?

I'd go with the oversize. The helicoil I don't think would pop out under pressure, but might leak, why take the chance if you have room to go oversize?
Gilly

Keensert
 

I would put a Keensert in it. This will give you the original size thread and will not leak. Provided there is enough material to install the Keensert.

I thought about this all night and decided to tap the hole to 12mm. The keensert looks great but I don't think there is enough room to countersink it. The tread depth is only about 10/15mm so I wouldn't want to take away any strength.

Thanks for the input. A lot has been going on here and I couldn't think straight. I didn't even think about the helicoil leaking - Doouuuhhh!

You could always use a sealant on the helicoil threads, but if it's just a plug, why mess with it, y'know.
Gilly