Understanding Pipe Thread Types

Ever tried to replace a fitting that almost screws in but won't seal? You've probably run into a thread compatibility issue. Not all threads are the same, and using the wrong type leads to leaks, stripped threads, and wasted time.

In our latest video, Darrin walks through how tapered NPT threads seal metal-to-metal with sealant, how straight threads use O-rings instead of thread sealant, and the critical difference between British BSPT and American NPT (they look identical but have different thread angles and are NOT interchangeable).

NPT (National Pipe Thread) 

NPT is the North American standard and used on most metal pipe fittings and plastic pipe fittings. These are tapered threads—the diameter gets wider as you screw the fitting in. The seal happens through metal-to-metal contact between the threads. 

• Key point: Always use Teflon tape or pipe dope on NPT threads. Without sealant, they'll leak. 

• How to identify: Visible taper, no O-ring, threads get progressively tighter. 

Straight Thread (with O-Ring)

Straight threads have no taper—the diameter stays consistent. The threads just hold the fitting in place. The actual seal comes from an O-ring at the base. 

• Key point: Do NOT use thread sealant on straight threads. The O-ring does the sealing work. 

• How to identify: Parallel threads, visible O-ring, fitting spins freely until it contacts the O-ring. 

• Common example: DI tank heads use straight threads with O-ring seals. 

British Standards: BSPP and BSPT

British threads follow different standards than US threads. 

• BSPP (British Standard Parallel) works like straight threads—parallel diameter, seals with a bonded washer. 

• BSPT (British Standard Tapered) is where confusion happens. BSPT is tapered like NPT and looks almost identical. But they're NOT interchangeable. 

• NPT has a 60-degree thread angle 

• BSPT has a 55-degree thread angle 

If you try to force NPT into BSPT (or vice versa), the fitting will bind up or you'll strip the threads. 

How to Tell Them Apart 

For NPT vs. BSPT, use a thread pitch gauge or caliper. They look nearly identical to the eye. 

• Pro tip: Roll the fitting across a flat surface. Tapered fittings (NPT/BSPT) roll in a circle. Straight threads roll in a straight line. 

Quick Summary 

• NPT: Tapered, needs sealant, North American standard 

• Straight Thread: Parallel, O-ring sealed, no sealant 

• BSPP: British parallel, bonded washer 

• BSPT: British tapered, different angle than NPT 

Need Help Identifying Threads? 

Still not sure which thread type you have? Our team at Specialty Sales can help get you the right match. 

Contact our sales team or browse our complete selection of pipe fittings and adapters.