Not long ago, one of my shop mates salvaged a junkyard engine. The only problem found was that one spark plug hole was cross-threaded. The labor cost to pull the head and repair it with a hardened thread insert was more than the value of the car; a moment's carelessness by a mechanic or the owner was the end of the road.
Fortunately, my buddy was assisted by a highly skilled pro, and they were able to do the job properly, with the only cost being their time. But if the insert ever sticks to a spark plug instead of the head, it will be a big problem. I've seen someone doing the same job on an outboard motor. He was scared, because marine parts always cost two or three times what an equivalent automotive part does. Both situations would have been reduced to a minor inconvenience by the Starr Products Back-Tap, which was invented by a technician who had similarly "oopsed" a spark plug hole on a Jaguar XK120.
I have cross-threaded other holes, or been around when someone else did, and had the joy of correcting the situation with a conventional tap, trying to distinguish the true threads from the cross threads. With a lot of care, a little skill and some luck, I've always gotten it right, but I've almost never been sure I would at the beginning.
The Back-Tap begins chasing good threads at the bottom of a hole rather than damaged threads at the top. Machined from heat-treated 4130 steel, it compresses smaller than the thread's inside diameter to be inserted into the hole. A mandrel is drawn into the tool with a knurled nut at the top, expanding the threaded portion. Then the expanded Back-Tap is threaded out through the hole. Filings are brought up and out of the hole along with the tool; greasing it before starting ensures that even the smallest shaving is retained, as well as making it easier to turn. Go in steps, leaving the Back-Tap slightly compressed at first, subsequently expanding it until the damaged portion is clear to the same diameter as the rest of the hole. Tolerances on spark plug holes vary, and some may be too tight if the Back-Tap is fully expanded.
The Back-Tap is sold through tool trucks and NAPA stores, in 10mm, 12mm, 14mm (the most common spark plug size) and 18mm. The latter is extra short, for use on O2 sensor ports. Hopefully, your need for the Back-Tap is rare enough that it may not be worth having in your toolbox all the time. But if you ever need it, you now know it exists and where to buy one.
