GlossaryKind of term: Standard and conventionPipe thread (G, R, NPT)
What is pipe thread?
A pipe thread is a thread for pipework and ports: parallel as G, tapered as R and NPT, each sealing in its own way.
A pipe thread is a thread for pipes and ports whose size designation refers to a nominal pipe size, not to the diameter of the thread. G1/4, for example, has a major diameter of just over 13 mm and 19 threads per inch. Three families appear on drawings: parallel G, the ISO 7 family with tapered external R and internal Rc (tapered) or Rp (parallel), and the American NPT. In the UK, G is usually called BSPP and R BSPT.
What does the standard say about pipe thread?
G follows ISO 228-1: parallel, with a 55° thread angle, and not pressure-tight on the thread; the seal sits on a face, a washer or an O-ring. R follows ISO 7-1: tapered 1:16 on diameter and sealing on the thread itself, usually with a jointing compound. Rc is the tapered internal thread, Rp a parallel internal thread used with a tapered R external thread. NPT follows ASME B1.20.1: also tapered 1:16, but with a 60° thread angle and, at most sizes, a different pitch. For hydraulic ports with G threads, ISO 1179-1 sets out the port and its sealing face.
How does pipe thread work in practice?
Pipe threads need tooling of their own: a G tap, a tapered R or NPT tap, or a thread mill with the right profile. The tapping drill depends on the form, and a tapered hole is sometimes taper-reamed first. A tapered thread is checked with a taper gauge: the step or notch shows whether the thread sits at the right depth, which a caliper cannot tell you. A G port often comes with a sealing face that has its own roughness and flatness requirement, or a groove for an O-ring. That feature does not follow from the callout, so look for it separately on the drawing.
What is pipe thread often confused with?
G is not R: both have a 55° thread angle and the same pitch per size, but G is parallel and seals outside the thread, R is tapered and seals on the flanks. NPT is not R: the thread angle differs, and at most sizes the pitch as well. Nor is a pipe thread a metric fine thread: G1/8 and M10x1 are close in diameter and pitch, yet they do not fit each other.
How do you see pipe thread in Blake?
In Blake a callout such as G1/4 or 1/4-18 NPT sits with the part as a proposal, written as on the drawing. A correction stays visible with name and time, and with a note if you add one. More on requests.
Frequently asked questions
What is the difference between G and R threads?
G is parallel and seals with a face, washer or O-ring outside the thread. R is tapered and seals on the thread itself, usually with a jointing compound. The profile is the same, but in a pressure-tight port they are not interchangeable without agreement.
Can I cut NPT with an R tap?
No. NPT has a 60° thread angle and R 55°, and at most sizes the pitch differs too. The parts may screw together and still leak. An NPT port needs an NPT tap or a thread mill with that profile.
What does G1/4 mean in millimetres?
G1/4 refers to a nominal pipe size, not to the thread itself. The major diameter is just over 13 mm, with 19 threads per inch. The sizes are in ISO 228-1; take the tapping drill from the tap or tooling supplier's table.