It can happen to the best of us: one moment of inattention results in a cross-threaded spark plug hole. You can either install a helicoil and hope for the best, or send it out to have the whole shebang re-tapped. Now you have another option. The Back-Tap can fix that cross-threaded hole from the inside — without pulling the head.
The Back-Tap is a tap with expandable threads and a mandrel to catch errant metal shards. Apply a dab of grease to the mandrel, then place the device in the hole past the damaged threads. As you back it out, the cutters expand out to engage the undamaged threads, then gently reshape the damaged ones. The greased mandrel keeps metal filings from ending up in the cylinder.
The result: a task that could potentially take days and cost hundreds of dollars is done in minutes for less than a hundred bucks.
MAC offers a three-piece kit that includes a 10mm, 12mm, and 14mm Back-Tap for $90. You can buy Individual taps from various sources for about $50 each.