1996 LT1. Previous owner made a mess trying to fix a broken exhaust manifold bolt. Got it cleared up, but the threads are really bad. Is it possible to redrill and thread for M12 threads? Yes, I know they are SAE from the factory, but I am in Europe and metric sizes are much easier to find studs/bolts for.
The outer diameter of the typical helicoil insert used to fix 3/8-16 seems to be .472", or 12 mm diameter, so basically drilling for a helicoil insert would be almost the same as drilling for a M12 bolt.

This leaves me thinking that redrilling the holes at 11,5 mm to make M12 threads should not be a problem as long as I take care not to drill any deeper than stock?

Any ideas? I definitely don't want to puncture a water jacket.