I've collected quite of a few barb fittings for my 00vi project and recently found a huge set of 1/4, 3/8, and 1/2 18-NPT taps, i was wondering how i could tell the pitch of my barb fittings to determine if i can use the 18 pitch taps.

I know there's a 14 NPT and a 18 NPT, so thats why im wondering. Is there some special tool?

 

A thread pitch gauge.

For NPT they seem to be called thread ring gauges:


A bit pricey, though: http://www.drillspot.com/products/44...read_Ring_Gage

I'd go to a machine shop, see if you can get one of the guys there to spec your fittings. That or somewhere that does a lot of pipe/plumbing work.

 

I'm hoping that what you're calling an "18-NPT" tap, is actually a 1/8"-NPT tap. If it is the smallest of the bunch then that's probably the case. Both 1/4" and 3/8" NPT threads have a pitch of 18 which makes the 3/8" look like a "fine" thead when compared to the other ones.

Another good way to tell what you have, short of finding the gauges that pmohr suggested, is to use some fittings of known size and try them. About the only tricky one is a 1/8"-BSPT (British Standard Pipe Thread) which uses a pitch of 28 instead of the 27 used in this country.

 

No. I typed it correctly, i have 1/4, 3/8, and a 1/2 18-NPT tap. Exactly how its printed on the bit.

 

Well, then, maybe it's labeled incorrectly. A 1/2" NPT thread usually has a pitch of 14. Fine and coarse 1/2" threads are 20 and 13 respectively. Hold the mystery tap up to a ruler and count the number of "teeth" (on one flute) that appear in an inch.

If it is an 18 pitch then it has to be either 1/4 or3/8" NPT. Check out this excerpt from the McMaster catalog: http://www.mcmaster.com/#taps/=2xpxav