Pipe Tap Guide: PT, PF, G, Rc Threads & Accuracy
Pick the wrong pipe tap and the thread cuts fine but the joint still leaks. This guide settles three things: whether your fitting needs a parallel or a taper thread, which standard the callout on your drawing belongs to, and how the internal and external threads pair up. TPI conversion, thread tolerance, and where JIS and ISO disagree on the same thread are all covered, so you can read the drawing and order the right spec.
1. Two Pipe-Thread Families
| Family | Codes | Traits |
|---|---|---|
| Taper pipe thread | PT, Rc (int), R (ext) | Taper form, seals by flank interference; gas and liquid piping |
| Parallel pipe thread | G (ISO), PF (JIS), Rp (int) | No taper, mechanical joint; needs a gasket/seal |
2. Thread-to-Tap Pairing
| Thread | Pairing |
|---|---|
| Taper | Rc → internal tap; R → external; PT → JIS standard |
| Parallel | Rp / G → internal; PF → parallel thread |
Uses: pipe fittings, valves, hydraulic systems.
3. JIS vs ISO Standards
| Item | Note |
|---|---|
| JIS standards | B0202 (parallel pipe), B0203 (taper pipe) |
| ISO correspondence | G matches Rp; Rc corresponds to PT |
| Differences | JIS and ISO differ slightly in size/accuracy; some specs merged over the years |
4. Pitch and Conversion
P = 25.4 ÷ n (n = threads per inch, TPI)
Pipe threads are given in TPI; convert to metric pitch (mm) with the formula for imperial/metric comparison and parameter setup.
5. Accuracy and Tolerance
Pipe-thread accuracy affects sealing, machining accuracy and assembly, controlled mainly on major, pitch and minor diameters. Taper threads seal by taper and tapping depth; parallel threads seal with a gasket. See the pipe-thread tolerance guide for detailed values.
For accuracy detail see the Pipe Thread Tolerance Guide; for troubleshooting see the Pipe Tap Troubleshooting Guide; for codes see the Thread Designation Chart; for pilot holes see the Special Thread Tap Drill Chart.
FAQ
Q: How to tell PT, Rc, G and PF apart?
PT/Rc are taper pipe threads that seal by flank interference; R is the matching taper external. G (ISO)/PF (JIS)/Rp are parallel pipe threads — no taper, needing a gasket. Choose taper or parallel by whether self-sealing is required.
Q: Are taper and parallel pipe threads interchangeable?
Not advised. Taper (PT/Rc/R) seals by flank interference and parallel (G/PF/Rp) by gasket; their forms and sealing differ. Mixing leaks or won't tighten — pair internal/external and taper/parallel correctly.
Q: Are JIS PT/PF the same as ISO Rc/G?
They correspond but aren't identical: ISO G matches Rp and Rc corresponds to JIS PT, but sizes and accuracy differ slightly. State the standard (JIS B0202/B0203 or ISO) for cross-standard purchasing.
Q: How to compute pipe-thread pitch?
Use P = 25.4 ÷ n (n = threads per inch). Pipe threads are given in TPI; convert to metric pitch to compare and set machining parameters.
For the full reading guide on this topic, see Thread Machining: A Complete Reading Guide.
Published: 2026-03-24 | Last updated: 2026-10-31
