One machined part that crosses the two systems — Metric ↔ UNC, UNF, BSP and NPT. Male or female each end, parallel or tapered, sealed for pressure. Because metric and imperial threads never fit each other, an adapter is the only correct fix.
Metric and imperial threads differ in diameter, pitch and thread angle — a metric 60° profile against a Whitworth 55°, a 1.5 mm pitch against 20 threads per inch. Even where two sizes look almost equal, screwing them together cross-threads and strips both parts. The only correct fix is a machined adapter that carries the metric form on one end and the imperial form on the other, each cut to its own standard.
This is the same job as our BSP-to-NPT and custom metric adapters, widened to every metric↔imperial crossing. Need the same system both ends? See male-to-male and female-to-male adapters.
| System | Threads | Standard | Seals by |
|---|---|---|---|
| Metric | M coarse & fine, M6–M48 | ISO 261 / ISO 68-1 / DIN 13 | Washer / O-ring / face |
| Unified inch | UNC, UNF, UNEF | ASME B1.1 | Washer / face |
| BSP parallel (G) | BSPP, G1/8–G2 | ISO 228-1 / BS EN ISO 228 | Bonded washer / O-ring |
| BSP taper (Rc/R) | BSPT | ISO 7-1 / EN 10226 / BS 21 | Thread (self-seal) |
| NPT / NPTF | 1/8"–2" American pipe | ASME B1.20.1 / B1.20.3 | Thread (self-seal) |
| Whitworth | BSW / BSF (legacy) | BS 84 | Washer / face |
Any one on the left joined to any other, in the gender and sealing form your joint needs — that is the whole point of a made-to-order adapter.
| Material | Density | Why choose it |
|---|---|---|
| Brass (CZ121 / C360) | 8.5 g/cm³ | Default — free-machining, seals well, low cost |
| Stainless 304 / 316L | 7.9–8.0 g/cm³ | 304 the common all-round grade; 316L for marine, chemical, washdown; 303 free-machining on request |
| Steel (zinc-plated) | 7.85 g/cm³ | High-pressure hydraulic strength, low cost |
| Aluminum 6061 | 2.70 g/cm³ | Light, low-pressure, weight-sensitive |
| Titanium Gr5 | 4.43 g/cm³ | Light + corrosion-proof, aerospace and premium |
An NPT-to-BSP adapter, tapered NPT one end (ASME B1.20.1), parallel G1/2 with a bonded seal the other (ISO 228) — the classic transatlantic mismatch, machined in brass or 316L.
A UNF-to-metric adapter, male M14×1.5 into the port, female 7/16" UNF for the transducer, sealed on an O-ring — machined to ASME B1.1 and ISO 261.
Whitworth-to-metric adapters bridge the eras: we cut the 55° BSW form to BS 84 one end and the metric to ISO 261 the other, from your sample if the size isn't marked.
Not sure of the two threads? Send the two parts, or a photo with a rule across the thread, and we identify both — metric, UNF, BSP or NPT — then machine the adapter that joins them. It's the same measuring we do for parts from a sample.
A few metric-to-imperial pairs are stocked. Most are not, because the two systems have no common thread form to build a range around.
| What you need | In the catalogue? | How we machine it |
|---|---|---|
| A common metric-to-BSP or metric-to-NPT pair | Sometimes stocked | Check a stockist first — if they list it, buy it off the shelf. We are worth calling when the pitch, the material or the second standard is not on their page. |
| Metric M against UNF or UNC | Thin | Unified threads on instruments, sensors and US-built assemblies. Both ends cut to their own standard. |
| Whitworth BSW or BSF against metric | No | Pre-1970s British machinery. The 55° Whitworth form has to be cut, not approximated. |
| A metric size that looks like an imperial one | No | Close major diameters are the trap — they start and then bind. We cut both ends properly rather than forcing a near fit. |
| Metric fine pitch against an imperial thread | No | Fine pitches are where stock ranges end first. |
| Left-hand on either side | No | Not stocked in either system. |
| An imperial thread on a machine with metric ports throughout | No | One adapter rather than replacing the port. |
Thread systems above are the ones this page already covers.
The most common metric-to-imperial machine job — an M33×3.5 chuck onto a 1×8 TPI or 3/4×16 TPI headstock, or the reverse — lives on wood lathe spindle thread adapters.
Yes — a single machined part with a metric thread one end and an imperial (UNC, UNF, BSP or NPT) thread the other. Male, female or mixed gender, sealed for pressure.
UNC/UNF/UNEF (ASME B1.1), BSP parallel G (ISO 228) and taper Rc/R (ISO 7-1), NPT/NPTF (ASME B1.20.1), and Whitworth BSW/BSF (BS 84). Metric side to ISO 261 / DIN 13.
Different thread angle, pitch and diameter — forcing them cross-threads and strips both. A machined adapter is the only correct fix.
Yes — parallel threads seal on a washer, O-ring or face; tapered (NPT/BSPT) seal on the thread. We machine the right form each end.
The thread and pitch each end, male or female, parallel or tapered, the material — or the two parts you're joining. Quote in 12 hours.
Tell us the two threads and the material. Engineers reply in 12h.
// Qty & price
1 pc
Sample price
Confirm fit before a run
10–50
Unit price drops
Setup cost shared
100+
Best price
All tiers quoted upfront
One machined adapter joins Metric to UNC, UNF, BSP or NPT — sealed for pressure, in brass, 316L or titanium. Quote in 12 hours.