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midgetracer · 05-28-2021, 08:14 PM

I purchased a Snap On spark plug tap that is listed as 7/8 18NS. Spark plug tap
Stock #T2818. I needed to clean up threads on a high compression head. The tap appears to be 7/8 18 tpi, but when I tried to clean up the threads, it cut more than I expected and the spark plug would not thread into the hole. I tried it on an old head I had laying around and it did the same thing. purchased it on EBay some time ago, so return is not possible. Any thoughts on what I have and what I need. I measured the tpi and it is 18.

05-28-2021, 08:14 PM	#1
midgetracer Senior Member Join Date: May 2010 Location: Bismarck ND Posts: 1,191	Thread size for a Model A spark plug I purchased a Snap On spark plug tap that is listed as 7/8 18NS. Spark plug tap Stock #T2818. I needed to clean up threads on a high compression head. The tap appears to be 7/8 18 tpi, but when I tried to clean up the threads, it cut more than I expected and the spark plug would not thread into the hole. I tried it on an old head I had laying around and it did the same thing. purchased it on EBay some time ago, so return is not possible. Any thoughts on what I have and what I need. I measured the tpi and it is 18.