Spark plug thread was crossed by prev owner

[schrader7032]

Nice point, Randy!

[sherman980]

Randy,
Assuming the damage is only to the top portion of the hole, I would be hesitant to run a tap through the hole from the inside as a first step. Couple of notes here. There are significant differences between a thread repair tap and a standard tap. Most repair taps that I am familiar with have a very long taper specifically to both help center the tap and to catch the "good" threads, if there are any, lower in the hole. Some even have an unthreaded portion that has an OD just the size of the ID of the threads specifically to prevent an "off-center" start. Most standard taps have a limited taper and are far more difficult to start in an already threaded hole. Also, most repair taps are designed to tolerances that are slightly smaller than the thread they are designed to fix. That way, they remove as little material as possible during the repair. Standard taps come in a range of tolerances and, unless you know what you have, you may end up removing material from the "good" area that you didn't mean to. That said, if Plan A doesn't work (repair tap from the outside in), then Plan B (repair tap from the inside out) wouldn't be an unreasonable step.

[Randolph]

Chuck,
The thing I always fear is starting a tap from the outside whose threads aren't going to line up with the "good" threads farther in. I did a search for "thread repair tap" and found several cheap double ended (M10 and M14) spark plug thread chasers and they have a round unthreaded nose. But they look rather short. Depending on the level of damage maybe that's all Keith needs.
I also checked McMaster and they don't have any specialty taps like you described. I wonder if a standard taper tap would work. Again it depends on the level of damage.
I also found a little longer repair tap here:

https://www.autozone.com/test-scan-and- ... lsrc=aw.ds

I'll be watching this thread to see what Keith comes up with.

-Randy

[Jim D 5112]

There are thread chasers made for spark plug holes that are inserted into the hole and then expanded like a blind hole collet puller. You put it through the hole then you can expand it to pick up the undamaged threads and chase the threads from the underside and back up to the top side. MAC made mine but I would think others are carried by Snap On or Matco.

[cbclemmens]

Even if you use a helicoil pull the head. It just isn't that much work, the gaskets are not expensive and you can clean all the cuttings out of the hole without any getting into the engine.

Craig