Forgotten.... What thread size are head studs into the block ?

[Phil Waterman]

Hi Richard

I like the 1/2 diameter spiral wire brushes in a drill at low speed to clean stud/bolt holes as a first step in cleaning threads. Soak the threads with the acetone and ATF over night or a couple of days then run the spiral wire brush in with the drill at slow speed when it bottoms reverse and run it out. I buy the cheap ones when I see them at flea markets.

Then as Lynn suggests use a worn tap of the correct size.

Which gets back to your original question what size? From time to time I have seen thread chaser sets which are intended for this purpose to clean but not really cut into the threads.

Cheers Phil