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midgetracer · 05-30-2021, 03:59 PM

The tap is labeled Snap ON 7/8 18NS. It measures 18 TPI and so does the C4 spark plugs I am using. It must have a taper of some sort that it will not turn in a stock Model A head. The threads on my HC head were slightly wrecked on one cylinder, so I bought this tap to clean them up. Waste of $45 and now I have a spark plug hole that really needs help. JW is your tap the correct one for a Model A and labeled 7/8 18 NF. I will look for one with that designation. My machine shop could not help me. Thanks for all the comments given.

05-30-2021, 03:59 PM	#15
midgetracer Senior Member Join Date: May 2010 Location: Bismarck ND Posts: 1,190	Re: Thread size for a Model A spark plug The tap is labeled Snap ON 7/8 18NS. It measures 18 TPI and so does the C4 spark plugs I am using. It must have a taper of some sort that it will not turn in a stock Model A head. The threads on my HC head were slightly wrecked on one cylinder, so I bought this tap to clean them up. Waste of $45 and now I have a spark plug hole that really needs help. JW is your tap the correct one for a Model A and labeled 7/8 18 NF. I will look for one with that designation. My machine shop could not help me. Thanks for all the comments given.