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CT Jack · 01-05-2020, 09:53 AM

A 1/4-18 NPT (National Pipe Thread) is used to thread a 1/4" inside diameter pipe. A 1/4" NPT tap will produce a 1/4" tapered thread with 18 threads per inch. US pipe tap sizes are sized by the inside diameter of pipe, not the the OD (outside diameter).

01-05-2020, 09:53 AM	#19
CT Jack Senior Member Join Date: May 2010 Location: Hebron, CT Posts: 432	Re: Shut off thread A 1/4-18 NPT (National Pipe Thread) is used to thread a 1/4" inside diameter pipe. A 1/4" NPT tap will produce a 1/4" tapered thread with 18 threads per inch. US pipe tap sizes are sized by the inside diameter of pipe, not the the OD (outside diameter).