"Chasing" generally involves following extant threads that have been damaged or filled with rust with a threading tap. It's re-tapping. There is no other form of re-tapping.

The reference to UV degradation has nothing to do with black-body radiation. It has to do with plastics and other substances breaking down on exposure to oxygen in the presence of ultraviolet light.

 

I have never heard of "re-tapping". If the thread form is damaged enough that a tap can be used again to remove additional material, there will be voids left in the resulting thread form. It will still be damaged afterwards, and thus has not been tapped properly.
Chasing is removing foreign material from the existing thread-form. Chasing is not tapping (cutting new threads in a drill hole) or "re-tapping".

Where would UV light come from in an engine bay? The only sources I can think of are spark plugs or HID headlight bulbs, and they're all contained and/or directed away from the engine bay.

 

Chasing is never performed with a tap which exactly matches the original tap; it will always (100%) be either larger or smaller than the original. More than 50% of the time the chasing tap is larger than the original tap (because intelligent mechanics and other repairmen never buy interference-fit taps). Therefore, the tap used for chasing either cuts or forms. Either is "tapping".

But you do you. I'm fine with it.

Where would UV light come from in an engine bay? The only sources I can think of are spark plugs or HID headlight bulbs, and they're all contained and/or directed away from the engine bay.[/QUOTE]

The engine bay is washed with UV light, just like the rest of our world is. Sunlight and skylight reflected off the ground, off puddles, off snow contain oodles of UV because UV reflects just like visible light does.

Really.

But - again - you do you.