BSPP, BSPT, R, and Rc Threads: Sealing and Compatibility Guide

BSP is a thread family, not one single connection. BSPP, BSPT, R, and Rc all relate to British Standard Pipe threads, but they do not seal or mate the same way.

BSPP Parallel Threads

BSPP threads use a parallel 55 degree Whitworth form. The thread holds the fitting in place, while the seal is made with an O-ring, bonded washer, face seal, or similar sealing element.

Because there is no taper, tightening the thread does not create thread interference. The port face, washer, O-ring, and fitting shoulder control the seal.

BSPT Tapered Threads

BSPT threads taper at approximately 1:16. As the male thread enters the female thread, the diameter change creates radial contact pressure.

This style often relies on metal contact plus sealant to fill small spiral leak paths. Too little engagement can leak, while too much torque can damage the port.

R and Rc Designations

R refers to an external tapered male BSP thread. Rc refers to an internal tapered female BSP thread. Both are part of the tapered thread family and are commonly associated with ISO 7-1.

R into Rc is a normal tapered-thread combination. R into a parallel female form may be conditional depending on the design. BSPP into a tapered Rc port should not be treated as a safe substitution.

Field Identification Steps

Measure the male outside diameter, count the thread pitch, and check whether the diameter changes from the first thread to the last thread. A changing diameter indicates a tapered thread.

The most common mistakes are assuming R means BSPP, ignoring where the seal occurs, mixing parallel and tapered ports, and over-tightening tapered threads. Selection should be based on pressure, reusability, maintenance frequency, and sealing reliability.