BSP/NPT thread identifiers

[imark]

Is there any marking or something obvious that I can use to identify the thread type on my kit?

[Dr Horrible]

Measuring the number of threads per inch (or mm) and comparing against a standard table is one way, but I had to do this at work one time and found that the simplest way is to hold up the thread to the light side-on so that you can see the peaks and troughs of the thread.  BSP has a rounded peak(and trough) whereas NPT is more angular and has a flat section at the top of the peak so doesn't come to a point.  This is why even when some sizes with the same number of threads per inch (1/2in, if I remember correctly) don't fit together for more than a turn or two - the shapes don't mesh together that well.

[imark]

Was hoping it would be a stamp on the fitting or something.

[Hop Bomb]

Ive a mix of both all over my setup. PTFE tape & they fit nicely. No issues. I think there is only half a thread in the difference (WillD posted that a while back)

[St. Fursey]

The 1/2 inch size pretty much interchangeable as the thread count per inch is the same -14 from memory. Other sizes become tricky and could lead to leaks or thread damage. +1 on ptfe tape

[Hop Bomb]

Ive only ever used half inch. Never even thought of the other sizes. Good to know.

[Will_D]

Both 1 /2" and 3/4" are 14 tpi.

There is a slight difference in diameter due to the different thread angles: 55 degrees for bsp and 60 degrees for NPT. However the taper of the fitting allows them to fit together. Of the two combinations: Male BSP Femal NPT and Male NPT into Femal BSP, One is tighter than the other. Always use teflon tape!

For the techies:

NPT versus Bsp:
Npt:
Series   Nominal Size   Pitch Tpi   Pich in " & mm
1⁄16   0.3125 in (7.94 mm)  27          0.03704 in (0.94082 mm)
1⁄8   0.405 in (10.29 mm)     27          0.03704 in (0.94082 mm)
1⁄4   0.540 in (13.72 mm)     18          0.05556 in (1.41122 mm)
3⁄8   0.675 in (17.15 mm)     18          0.05556 in (1.41122 mm)
1⁄2   0.840 in (21.34 mm)     14          0.07143 in (1.81432 mm)
3⁄4   1.050 in (26.67 mm)     14          0.07143 in (1.81432 mm)


Bsp:
Series   Nominal Size   Major Dia in   Major Dia mm   Pitch tpi   Pitch mm
BS Pipe   G 1/16           0.30400           7.72160               28   0.9071
BS Pipe   G 1/8           0.38300           9.72820               28   0.9071
BS Pipe   G 1/4           0.51800         13.15720               19   1.3368
BS Pipe   G 3/8           0.65600         16.66240               19   1.3368
BS Pipe   G 1/2           0.82500         20.95500               14   1.8143
BS Pipe   G 3/4           1.04100         26.44140               14   1.8143

[Dr Horrible]

Good to know that 1/2" works for a brew, but I can guarantee you it doesn't seal at higher pressures! You can buy thread gauges on line that measure the thread, the cheap ones are just a set of 'keys' with the thread profiles outlined on them, they're only a few euro (or used to be anyway).  However, if you're dealing with 1/2" where the threads per inch is the same, visually checking the profile is the best way - try it, it's easy enough, particularly if you have both types to compare against each other.  And it will be a great new trick to impress your friends down the pub!