Pipe Tap Guide: PT, PF, G, Rc Threads & Accuracy
This guide organizes pipe-tap categories and their thread standards: parallel pipe threads (G/PF), taper pipe threads (PT/R/Rc), their internal/external pairing, plus JIS vs ISO differences, TPI conversion and tolerance. A systematic cross-reference lets you quickly pick the right pipe tap and spec — widely used in fluid sealing, pneumatic and hydraulic systems, a key basis for CNC machining and pipe-fitting design.
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.
