I like the spiral wire brushes as first step
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
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