Working on 65 valiant it is missing 2 exhaust studs 1) can they be drilled out in the car & 2) what size bit is needed? Will this mess up the cast iron head? The exhauxt leak gets louder each time I drive it, I'm either going have to fix it or invest in a nice sound system(not exhaust notes)

The head end of the stud is 5/16" coarse thread and the drill size would be 1/4". If you have a steady hand you should be able to do it in the car. With the manifolds off it is pretty wide open.

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Official Cookie and Mater Tormentor.

I prefer an air drill if the engine is in the car because it is smaller and easier to control.

Wrong question.

The right question is "What size bottle of bourbon is needed to drill out two broken manifold studs?".

take a center punch and get it good and center before you start... I also like to use left hand drill bits because somtimes between the center punch breaking loose some of the rusted threads and the left hand bit putting stress in the out direction if the bit catches you dont' even need the easy out.
SOMETIMES!!!