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Mon Mar 02, 2009 10:36 am

Can one of you kind Americans explain to a metric devotee from across the pond what gives a 3/8" NPT that size description.

Doesn't seem to measure 3/8" to me...

Mon Mar 02, 2009 10:51 am

fredster wrote: Can one of you kind Americans explain to a metric devotee from across the pond what gives a 3/8" NPT that size description.

Doesn't seem to measure 3/8" to me...

Don't blame us. The English gave us our measuring system!

I think the 3/8" NPT was supposed to indicate what inner diameter pipe the threads were meant to be cut on. It's a trade size, not an actual thread dimension.

Scott

Mon Mar 02, 2009 10:54 am

It is the inner diameter of the pipe.

Mon Mar 02, 2009 11:27 am

"Don't blame us. The English gave us our measuring system!"

We were just to pig headed to change to metric when we had the chance. Now we drive cars that are part fractional and part metric and lot's of fun to work on.

Mon Mar 02, 2009 12:19 pm

Ha!

The Imperial measurement system is our revenge for booting us Brits out of power during the War of Independence.

That'll teach ya.

Mon Mar 02, 2009 1:13 pm

all you needed to know but were afraid to ask...
http://en.wikipedia.org/wiki/National_pipe_thread

the British use BSP which is a little different ( thread pitch etc.)

Mon Mar 02, 2009 2:25 pm

Ha, that's something I wanted to ask my teacher. Every time NPT sizes come up in any conversation the only thought I am left with is who ever came up with the standard had a complete brain fart.

It seems like the only way to know what size you have in your hand is by knowing what you have. To make a positive ID of the size you have to measure OD and number of threads, then refer to a chart.

Quote: Note! Pipe sizes do not refer to any physical dimensions. The outside diameter of a pipe or fitting must be measured and compared to a table for size identification. A 3/4" NPT pipe thread has an outside diameter - OD - of 1.050 inches.

http://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html

Anton

Tue Mar 03, 2009 8:16 am

I always end up referring to the boards with the fitting ends mounted on them at Lowe's for pipe fitting sizes. There's probably such a thing as an NPT thread gauge, but you'd only use it about twice in a lifetime. I decided to wait until the stuff for my full flow setup came in the mail before I ordered the fittings, good thing too because I would have ordered the wrong ones.

Tue Mar 03, 2009 8:23 am

fredster wrote: Can one of you kind Americans explain to a metric devotee from across the pond what gives a 3/8" NPT that size description.

Doesn't seem to measure 3/8" to me...

"think automotive" is a great place to buy fittings from and in their catalogue, they also have an explanation as to the differences between the various threads they do.

Try to avoid the aluminium ones though, the wall thickness is larger than the steel ones and they cost a fortune in comparison.

They weigh F'all though :D

As a comparison, 1/8 npt has 27 threads per inch whereas BSP has 28 and metric has 25 and a bit at 1mm pitch.

I believe 3/8" inch in npt is the internal thread diameter, not external at half taper but i could be wrong :?

Tue Mar 03, 2009 11:59 am

Bugorsh wrote: It is the inner diameter of the pipe.

You're correct; the size was originally a thread size so you could have a pipe that's 3/8" diameter on the inside. Then they came out with a bunch of different wall thickness options for the pipe, so you can get 3/8" NPT pipe that isn't quite 3/8" on the inside. Pipe threads are a bit confusing.

Tue Mar 03, 2009 8:36 pm

It's all a conspiracy by plumbers to keep their vocation a specialized trade and not a DIY thing.

Tue Mar 03, 2009 8:41 pm

3/8 is the nominal inside pipe diameter. There are different shell thicknesses to meet different pressure ratings such as SCH10,SCH 40, SCH80 and so on.

If you really want to get confused take a look at refrigeration pipe sizes. They flip flop back and forth between inside diameter, outside diameter and just plain a arbritrary size that it makes your head hurt...