Choose a cable-entry thread by its complete designation, mating role and sealing arrangement. G and NPSM are parallel mechanical threads; R is an external taper thread that mates with the specified Rp or Rc internal thread; NPTF uses controlled thread contact for its Dryseal mechanism. A legacy PF marking also needs its original class and drawing context.
I start with the two surfaces that must mate and the place where water could pass. That separates thread fit from sealing. The nominal size then helps locate the right drawing or gauge; it does not describe the cable bore or settle which thread family fits the opening.
Key takeaways
- For G or NPSM, identify the separate face, seat or gasket that seals the joint.
- Keep R, Rp and Rc distinct: the letters specify both form and mating role.
- Preserve a legacy PF marking and class while comparing the replacement dimensions.
- For NPTF, follow the exact mating-thread and assembly requirements, including any permitted compound.
- Describe both ends of an adaptor and check its shoulder, bore and installed length.
Record the complete designation before translating it
I copy the marking exactly, including spaces, fractions, suffixes and any class or hand designation, then match it to the equipment drawing. This keeps the original identity available while we compare a replacement. In particular, retain the full NPSM series and any PF class rather than shortening them during a catalogue search.
Nominal pipe size is the size designation used within a pipe-thread family. The measured outside diameter and cable bore are separate dimensions. To identify a “half-inch entry,” add the governing standard, thread series, pitch or TPI as applicable, male/female role and usable length. Use the standard or drawing for the corresponding limits.
Where the equipment uses a plain opening and locknut, record the hole, wall thickness and locknut arrangement. The threaded-versus-plain entry guide explains the different mounting arrangement.
Distinguish G from R, Rp and Rc
G is the parallel pipe-thread designation associated with ISO 228-1. Its internal and external threads provide a mechanical connection; pressure tightness is achieved elsewhere in an appropriately designed joint. The ISO 228-1:2000 scope explicitly distinguishes this from making a pressure-tight seal on the threads.
R, Rp and Rc are the external-taper, internal-parallel and internal-taper designations in the pipe-thread system associated with ISO 7-1:1994. Select the permitted mating combination and assembly method from the governing specification. Although both G and Rp can describe an internal parallel form, their different jointing systems require different compatibility checks.
| Designation | Basic identity to preserve | Sealing question |
|---|---|---|
| G | Parallel internal or external pipe thread | Which separate face, washer or other designed interface seals the joint? |
| R | External taper pipe thread | Which specified internal counterpart and assembly condition apply? |
| Rp | Internal parallel jointing pipe thread | Is the required R counterpart and its thread-sealing arrangement documented? |
| Rc | Internal taper jointing pipe thread | Is the matching taper joint specified and within its limits? |
An R/Rp joint combines a taper male thread with a parallel female thread as part of its specified jointing system. With a G arrangement, the separate seal needs a suitable seating surface outside the thread. I locate that surface on the section drawing before choosing an O-ring or washer; the entry-seal comparison explains the different seating geometries.
The upper section below locates a seal beneath the shoulder. The lower section locates sealing contact along a specified taper joint. Compare those locations with the section drawing of the selected assembly.
The thread-family comparison concerns the gland-to-enclosure interface. The cable-to-gland interface also needs the correct sealing insert and cable diameter. When checking ingress protection, inspect both leakage paths and the enclosure assembly specified for the rating. Tightening the entry thread does not compensate for an incorrect cable-seal size.
Treat PF as a legacy identification task
PF appears in older Japanese parallel pipe-thread designations. The practical question is which original class and standard the equipment drawing used. Yamawa’s thread-reference note explains the historical relationship between G and PF and identifies removal of the old PF class-B internal-thread category. That class distinction matters when choosing a replacement for a legacy internal thread.
For a worn part marked only PF, I compare the original drawing or maintenance specification with the proposed replacement. Check the original class, finished dimensions, coating condition and usable engagement. The resulting comparison identifies the applicable gauge basis and whether the new part preserves the intended fit.
The same note distinguishes older PT taper designations from R external and Rc internal terminology, and PS internal parallel from Rp. Keep those historical clues in the inquiry, but describe the proposed new interface with its complete current controlled designation.
Separate NPSM, NPT and NPTF
NPSM is a straight pipe thread for free-fitting mechanical joints. NPT is a general-purpose taper pipe thread. Both belong within the families covered by ASME B1.20.1. The historical NBS screw-thread handbook, Part II, also identifies NPSM as a free-fitting mechanical joint. That older source supplies terminology here; current numerical limits require the governing specification.
With an NPSM connection, the specified seat, gasket or other separate interface provides the seal. Inspect that part of the joint as well as its thread. Retain all four letters in the description: “NPS” can refer to nominal pipe size in other contexts and leaves the required thread series unclear.
NPTF is a Dryseal taper thread. ASME B1.20.3 explains that controlled crest and root truncations create contact that obstructs the spiral leakage path. The intended sealing mechanism depends on conforming, undamaged mating threads. Its public description also allows compatible lubrication or sealant in appropriate circumstances to reduce galling.
For NPTF, I select the exact specified mating pair and follow its preparation instructions. Check tolerance, gauging and reuse conditions together. Compatible lubrication or sealant can be permitted; the Dryseal name describes the sealing mechanism rather than a blanket ban on compound. A mixed NPT/NPTF pair needs its own applicable assembly requirements.
Identify an unknown cable-entry thread
Before removing an installed gland, isolate the equipment under the applicable site procedure. Begin identification with non-destructive observations: is the opening plain or threaded, is its accessible form parallel or tapered, and what marking or seat remains? Measure accessible diameter at stated axial locations, compare pitch or TPI and inspect the profile. Chamfer, incomplete threads, wear and coating can distort each observation.
A pitch leaf and caliper narrow candidate identities. Then use the relevant dimensional or gauge method to check the candidate against its limits. For a taper thread, the reference plane is especially important: a diameter without its axial location is incomplete. For the actual mating decision, the thread-conversion guide covers both interfaces and the installed adaptor stack.
For a measurement example, assume two candidate drawings specify 14 TPI and 18 TPI, and the part exposes six complete crests along its axis. The distance from the first crest to the sixth contains five pitch intervals. Using NIST’s exact 25.4 mm per inch conversion, the calculated spans are 5 × 25.4 ÷ 14 = 9.07 mm and 5 × 25.4 ÷ 18 = 7.06 mm. I found a 2.02 mm difference by comparing the unrounded results, which makes the two assumed pitches easier to distinguish than a single interval. Measure between equivalent crest positions and account for the method’s uncertainty and any damage. This calculation narrows the pitch candidates; the profile, taper, diameter and tolerances still determine the thread identity. Count intervals carefully: treating six crests as six intervals would shift both predictions.
If a test component catches and then binds, stop and inspect the pitch, profile, taper and damage. Wrench force can damage both parts. Where the original interface remains unidentified, evaluate a replacement entry plate or an engineering-approved redesign so the new entry has a known geometry and sealing arrangement.
Example: two openings both described as half-inch
For example, assume an enclosure drawing calls for a G 1/2 female entry with a flat sealing land. Its replacement gland drawing specifies the matching G 1/2 male thread and a shoulder washer. A second enclosure instead specifies an Rc 1/2 female entry and an R 1/2 male accessory. These are hypothetical drawing conditions, chosen to compare two different sealing arrangements at the same nominal size. For the first enclosure, I check whether the washer is supported around the opening and whether the shoulder reaches it with adequate engagement. For the second, I check the specified taper pair and its assembly requirements. Exchanging the accessories would change both the mating system and the intended sealing location. Recording only “1/2 inch” loses those distinctions before the parts even reach the bench.
An adaptor adds another pair of interfaces. It may resolve the thread mismatch while moving the gland shoulder outward, reducing cable bend space or hiding the wrench flats. Check the assembled section and available access as well as the two thread descriptions. The adaptor installation guide follows these two joints through installation.
Specify both ends of an adaptor
Define the equipment side and gland side as separate interfaces. Start with the mating role, size and standard at each end, then add the sealing and installed dimensions. This turns a broad “G to NPT” description into a part that can be compared with both drawings:
| Field | Equipment side | Gland side |
|---|---|---|
| Governing thread | Complete standard, series, size and revision | Complete standard, series, size and revision |
| Mating role | Male or female, with counterpart identified | Male or female, with counterpart identified |
| Dimensions | Usable engagement, reference plane and limits | Usable engagement, reference plane and limits |
| Sealing interface | Named seal/seat and preparation condition | Named seal/seat and preparation condition |
| Installed geometry | Wall/boss, recess and permitted projection | Shoulder, wrench space and cable access |
Add material and finish, minimum clear bore, any captive components and the drawing revision for the complete adaptor. A correct thread pair can still fail the layout if the shoulder lands on a counterbore edge or the added length consumes the cable bend space. Compare the installed stack before accepting the proposed drawing.
For hazardous-area equipment, select an accessory permitted by the actual certification and equipment instructions. Preserve the specified protection concept, entry arrangement and installation conditions when evaluating a replacement.
Conclusion: Preserve the full interface description
These pipe-thread names are the starting point for our interface record. G, R, Rp, Rc, legacy PF, NPSM and NPTF carry different information about form, mating role or sealing intent. The useful comparison includes the drawing, limits, reference plane and actual sealing location.
For a replacement or adaptor, keep the original marking and describe both mating ends explicitly. Then check the shoulder, bore, engagement and installed access together. The result is a replacement that matches the intended mechanical joint, leaves room for installation and puts the seal on its designed surface.
FAQ
Are G and Rp internal threads interchangeable because both are parallel?
No. G and Rp belong to different thread and jointing systems, with their own dimensions, tolerances and intended mates. I compare the governing drawings and sealing arrangement: a G joint uses a separate seal, while the specified R/Rp pair forms part of a thread-jointing system.
Can I replace an old PF marking with G on an order?
Only after the legacy standard, class and actual interface have been checked against the proposed G replacement. The old PF system included distinctions that a simple letter replacement can lose. I keep the original marking alongside the proposed replacement and compare the finished dimensions, limits and sealing arrangement.
Does NPTF mean that sealant is always prohibited?
No. Dryseal describes a controlled thread-sealing mechanism. ASME’s public explanation also discusses compatible lubricant or sealant where appropriate to reduce galling. Follow the instructions for the exact pair and application; the name alone neither requires nor prohibits a particular assembly compound.
Is a short hand fit enough to identify an unknown pipe thread?
No. Similar diameter can allow initial engagement despite an incompatible profile, pitch, taper or tolerance. I identify the thread from documentation and appropriate measurement or gauging. If a part binds, stop rather than using wrench force as an identification method.
Sources and technical references
- ISO 228-1:2000 — Pipe threads where pressure-tight joints are not made on the threads, accessed 2026-10-28.
- EVS — ISO 7-1:1994 — Pipe threads where pressure-tight joints are made on the threads, accessed 2026-10-28.
- Yamawa — Pitch Diameter of Pipe Threads, technical note 085, accessed 2026-10-28.
- ASME B1.20.1 — Pipe Threads, General Purpose, Inch, accessed 2026-10-28.
- NBS Handbook H28, 1957 Part II — historical pipe-thread terminology, accessed 2026-10-28.
- ASME B1.20.3 — Dryseal Pipe Threads, Inch, accessed 2026-10-28.
- NIST — SI Units: Length, accessed 2026-10-28.