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The tap drill size for 1/8 npt is a "Q". You would then tap it with a 1/8npt tap. "Q" tap size in decimals is .3320 inches. The npt taps are different from standard taps in that npt taps are tapered. Hope this helps.

John B.
 
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Ummm.... isn't 11/32 too big for a 1/8npt tap??

5/16 has worked perfectly for me. The tap I use recommends a 21/64 bit but I'd rather go a bit smaller and give the tap more metal to dig into.
 
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