Technical reference
Threads
Metric, Unified, Whitworth, pipe and trapezoidal threads: sizes, pitches and what interchanges.
ISO metric (M)
ISO metric threads are 60° V-threads designated by M, the major diameter in millimetres and, for fine pitches, the pitch in millimetres (M12 is coarse, M12×1.25 is fine).
- USA
- M12×1.75-6g
- UK
- M12
- EU
- M12, M12×1.25
- Indie
- M12
- Japan
- M12
UNC / UNF
UNC (Unified coarse) and UNF (Unified fine) are 60° inch threads designated by diameter and threads per inch, such as 1/2-13 UNC or 1/2-20 UNF.
- USA
- 1/2-13 UNC-2A
- UK
- 1/2-13 UNC-2A
- EU
- 1/2-13 UNC
- Indie
- 1/2-13 UNC
- Japan
- 1/2-13 UNC
BSW / BSF
BSW (British Standard Whitworth) and BSF (British Standard Fine) are 55° inch threads with rounded crests and roots, defined in the UK by BS 84.
- USA
- 1/2 in BSW
- UK
- 1/2 in BSW, 1/2 in BSF
- EU
- W 1/2"
- Indie
- 1/2" BSW
- Japan
- W1/2
BSP vs NPT
BSP pipe threads use the 55° Whitworth form, either parallel (G, ISO 228-1) or taper (R, Rp, Rc, ISO 7-1 and EN 10226), while NPT uses a 60° form tapered 1 in 16 to ASME B1.20.1.
- USA
- 1/2-14 NPT
- UK
- G1/2, R1/2, Rp1/2, Rc1/2
- EU
- G1/2, R1/2, Rp1/2, Rc1/2
- Indie
- R1/2, Rp1/2, Rc1/2 (IS 554); G1/2 (IS 2643)
- Japan
- G1/2, R1/2, Rp1/2, Rc1/2
Acme / Trapezoidal
Acme and metric trapezoidal threads are flat-topped power-transmission threads used for lead screws, jacks, clamps and valve stems.
- USA
- 1/2-10 ACME-2G
- UK
- Tr 20×4
- EU
- Tr 20×4
- Indie
- Tr 20×4
- Japan
- Tr 20×4