[chris_seven]

I would worry about re-cutting threads as you will almost certainly affect the 'fit' of the stud and in extreme cases the load carrying capacity.

The issue is that the tap will never quite pick up on the same lead it will always start cutting in a slightly different position to the original stud.

I am not sure this is always very important depending on the engine case and the stud and just how much 'fit' has been lost.

ARP, Supertec, Casper Labs and any other stud with an increased shank diameter will increase the force on the thread produced by the thermal expansion of the cylinder.

Whilst this increase is clearly not a problem on Aluminium cases and Mag cases with Case savers I would be concerned about the thread fit on a standard Magnesium Case and avoid re-tapping the case thread other than to fit Timeserts or Case Savers.

Even with a pug tap I am not sure you will be able to cut threads to the bottom of a blind hole.