[Sirrotica]

I had to look up "back tap" because I've never seen one. The idea apparently is to collapse the cutting edges, push the tool into the combustion chanber, expand it to cut the threads as you unscrew it out of the cylinder head. Basically tapping it from the inside out.

I've never seen , or heard of the tool, but I'm sure it would start straighter from the bottom, undamaged threads, so as not to cross thread if a conventional tap was run in from the top. I'm sure it has a limited use, and is usually being used to straighten out threads that were cross threaded inadvertently with a spark plug change. If the threads were pulled out of the head from overtightening, you're only choice would be to restore the threads with an insert, or a Heli Coil.

I have a tap that both ends can be used depending what size threaded hole your working on. It fits into a sparkplug socket so you use the socket as the tap wrench. I've used it a few times to fix the top threads when they got crossthreaded.

Wisconson engines are classic for people ruining the plug threads, they take a 14 MM plug, but are aluminum, as well as the theaded area is thin, so a very limited area is threaded. If you mess up likely you're looking at a new head, or removing it for an insert installation. Those engines usually are exposed to rain water, and it pools around the plug further causing corroision problems when the plugs are in there for many years. Removing a plug sometimes also means the threads are coming out with it.