I need to know how to replace a thread that is stripped, i got a set of heads for a future build and most of the holes from the exhaust side are heli coiled, and one of the screw holes on the exhaust side is all stripped, i guess someone tried fixing it and left it half way and now i have to fix that, any suggestions on how to re thread it, also how much would i be looking at to pay at a local machine shop.. thanks

also does anyone know the size this hole is..also would i be off using a heli coil again, what size and what tools do i need.