Will NPT fit BSP?

NPT and BSP are different threads. At 1/2" and 3/4" both have 14 threads per inch and are almost the same width, which makes them easy to mix up, but BSP threads are cut at 55° and NPT threads at 60°. The safe way to join the two is a BSP to NPT adapter with one of each thread.

How BSP and NPT differ

BSP (British Standard Pipe) comes in two kinds: parallel (BSPP, made to ISO 228-1) and tapered (BSPT, made to EN 10226-1, also known as ISO 7-1). Some makers write 1/2" BSP as G1/2. NPT is the American tapered pipe thread.

Both tapered threads have the same 1° 47′ taper. From 1" to 2" the thread counts differ too: NPT has 11.5 threads per inch and BSP 11.

Joining an NPT part to a BSP part

Neither Swagelok nor Conex Bänninger says a BSP part and an NPT part can be screwed straight together. To join them, use an adapter with one thread at each end. It’s named by its two ends: female (threads inside) or male (threads outside), and the thread on each. Swagelok makes the 1/2" pair as SS-8-AG-8, a 1/2" female ISO parallel (BSPP) x 1/2" male NPT, and SS-8-A-8RS, a 1/2" female NPT x 1/2" male ISO parallel.

Use a parallel BSP end where your part seals with a washer, and a tapered end where it seals on the thread. For a shower head and arm, see shower head thread size.

Parallel or tapered: tape or a washer?

Tapered threads, BSPT and NPT alike, are designed to seal between the threads, and they always need a sealant: PTFE tape or a pipe thread sealant on the male thread. For drinking water, use a tape or sealant listed for drinking water.

Parallel threads (BSPP) don’t seal on the thread at all. The seal comes from a washer, an O-ring or a metal seat pressed between the two parts, and sealant isn’t needed or recommended on the thread. Conex Bänninger supplies drinking water approved washers with its compression tap connectors and says to use them.

Some BSP joints mix the two. A tapered male BSP thread can go into a parallel female, but a parallel male doesn’t go into a tapered female. Conex Bänninger’s Series 8000 brass fittings are made that way, with parallel female threads and tapered male threads, both to ISO 7-1 and EN 10226-1, and the joint is sealed with PTFE tape or liquid sealant on the thread. For drinking water, Conex advises at least five rounds of PTFE tape made to BS 7786. Other makers give other wrap counts: see threaded fitting leaking.

How to tell which thread you have

Look on the part, its package or the maker’s sheet first for NPT, BSP, BSPP, BSPT or a G size. If nothing is marked, measure it.

1. Tapered or parallel: measure across the tops of the threads at the first, fourth and last full thread. If all three are the same, the thread is parallel. If they get wider from the tip of a male part toward the back, it’s tapered.
2. Size: measure across a male part’s threads (on a tapered one, at the fourth or fifth thread). About 0.83" (21 mm) is 1/2", and about 1.04" (26.4 mm) is 3/4". A caliper is best, but a ruler held across the end is enough to tell 1/2" from 3/4". For a female part, measure the male part that goes into it.
3. Thread count: count the threads over one inch.

Measuring can’t settle BSP or NPT at 1/2" and 3/4": the widths are less than 0.2 mm apart and the count is the same. BSPT and NPT at these sizes are so close in design that a positive identification can take an optical comparator. If you can’t tell, go by the markings, or ask the maker of the part.

For US pipe threads from 1/4" to 1", see pipe thread size, or use Identify it.

Turn the water off before you undo a threaded joint. This page covers water fittings only: What Adapter For doesn’t cover gas lines.