NPT Threads

NPT stands for National Pipe Taper, the American standard pipe thread on most pneumatic fittings sold in the United States. You will also see it written out as National Pipe Thread. The word that matters is taper. An NPT thread is cut on a cone rather than a cylinder, losing 1/16" of diameter over every 1" of length, or 3/4" per foot. As you tighten the joint the male and female flanks wedge against each other, so the threads make the seal themselves instead of clamping down on a washer.

Pipe sizes are nominal. The number in 1/4" NPT came from the iron pipe the standard was built around, where 1/4" described the bore rather than the outside. The thread was then cut into the outside wall of that pipe. What you can get a tape across is therefore always bigger than the name, so a 1/4" NPT male thread measures about 0.540" across the crests. Nothing is mislabeled. Treat the size as a name and read the real diameter off a chart.

Work from the male thread whenever you can. Measure across the outside of the threads at the large end, which on a male fitting is the open end. Calipers are best, but a tape measure is accurate enough to pick the size out of the list below.

• 1/8" NPT: 0.405" outside diameter, 27 threads per inch
• 1/4" NPT: 0.540" outside diameter, 18 threads per inch
• 3/8" NPT: 0.675" outside diameter, 18 threads per inch
• 1/2" NPT: 0.840" outside diameter, 14 threads per inch
• 3/4" NPT: 1.050" outside diameter, 14 threads per inch

Take the reading at the widest point and allow a few thousandths of an inch either way. A female port reads slightly smaller than the male thread that fits it, so the quickest check on a tank is to try a male fitting you have already identified. If two sizes still look close, count the threads instead.

Five sizes cover almost everything here: 1/8", 1/4", 3/8", 1/2" and 3/4" NPT, and every tank listing gives its own port count in the specifications. The 1 Gallon 5 Port Steel Air Tank has one 1/2" NPT port and four 1/4" NPT ports, while the 5 Gallon 8 Port Steel Air Tank has seven 1/2" NPT ports and one 3/4" NPT port. The 4 Gallon Pancake 8 Port Steel Air Tank carries all four sizes at once: four 1/2" NPT ports, two 3/8" NPT ports, one 1/4" NPT port and one 1/8" NPT port.

The leader hose and check valve is 1/4" NPT on most of our compressors, though the Viair 460C Chrome Air Compressor and the Viair 480C Air Compressor come with a 3/8" NPT braided hose. Accessories use the smallest sizes, so our Schrader valve is 1/8" NPT, safety blow-off valves are 1/4" NPT, and drain cocks come in 1/4" NPT and 1/2" NPT. Brass reducers bridge whatever is left over. The 2-Gallon Single Compressor Fitting Kit ships with either a 1/2" NPT to 1/4" NPT or a 1/2" NPT to 3/8" NPT brass reducer. The 12-Gallon Triple Compressor Fitting Kit adds a 3/4" NPT to 1/2" NPT reducer. All of it is in fittings and fitting kits.

BSPT is British Standard Pipe Taper, marked R on a male thread. It tapers like NPT and seals on the threads themselves, so the two can look like the same part in your hand. The thread form is where they part company. BSPT flanks meet at 55 degrees where NPT flanks meet at 60 degrees, and the pitch is different at most sizes. A 1/8" BSPT thread runs 28 threads per inch against 27 threads per inch on 1/8" NPT, and 1/4" and 3/8" BSPT run 19 threads per inch against 18 threads per inch on NPT. Only 1/2" and 3/4" BSPT share a pitch with NPT, which is how a mismatched pair can start by hand and then leak once pressure builds.

For ports that use it we stock four adapters: the 1/8" Male BSPT to 1/2" PTC Fitting, the 1/8" Male BSPT to 1/2" PTC Elbow Fitting, the 3/8" Male BSPT to 1/2" PTC Fitting and the 3/8" Male BSPT to 1/2" PTC Elbow Fitting. Each one threads into a BSPT port and takes 1/2" air line.

BSPP is British Standard Pipe Parallel, marked G. The parallel part is the problem. Its threads run straight down a cylinder, so nothing wedges as you tighten it. The seal is made at the face instead, by a bonded washer or an O-ring. Thread a BSPP male into an NPT port and there is nothing for that washer to seat against, so it leaks.

Yes. The taper pulls the flanks tight, but the spiral root of the thread still leaves a path for air, so every NPT joint wants PTFE tape or a liquid thread sealant to fill it. Two or three wraps in the direction the thread turns is enough, started a thread back from the end so nothing tears off into the system. Run the fitting in by hand, then about two turns with a wrench and stop, because brass strips easily. Plenty of our fittings and valves arrive with PTFE already applied, and the listing says so when they do.