Thread Identification & Pitch Chart
Identify a thread by diameter and pitch: thread form drawings, a metric / TPI pitch table with the near misses, sizes by measured diameter, and odd threads - UNEF, ME, BSB, cycle, Tr and Acme.
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How to identify a thread
- Measure the diameter of the bolt with a calliper (worn bolts measure 0.1 - 0.2 mm under). For a nut or tapped hole, measure the bore and add about one pitch.
- Find the pitch with a thread gauge - try metric and inch leaves, along 10 or more threads. No gauge? Measure across 10 threads and divide by 10 (mm), or count the threads in one inch (TPI).
- Check for taper - measure both ends. A 1 in 16 taper means a sealing pipe thread (BSPT or NPT).
- Look at the form - rounded crests = 55° Whitworth / BSP; sharp V with flat crests = 60° metric, UNC / UNF or NPT; flat-topped teeth = lead-screw thread.
- Look it up below. Pipe threads are named after the pipe bore, so a 1/2" BSP thread measures about 21 mm.
Pitch and TPI
Metric pitch (mm) - TPI in bracketsInch thread (TPI)
Red = a pitch from the other system within 4%. 1.25 mm and 20 TPI differ by only 1.6% - hold the gauge along the full length of the thread.
| Pitch mm | TPI | Used on | Close to (other system) |
|---|---|---|---|
| 0.25 | (101.6) | coarse M1, M1.2 | |
| 0.3 | (84.7) | coarse M1.4 | |
| 0.318 | 80 | UNF #0 | |
| 0.35 | (72.6) | coarse M1.6, M1.8; BA 10BA | 72 TPI (+0.8%) |
| 0.353 | 72 | UNF #1 | 0.35 mm (-0.8%) |
| 0.39 | (65.1) | BA 9BA | 64 TPI (+1.8%) |
| 0.397 | 64 | UNC #1; UNF #2 | 0.39 mm, 0.4 mm (-1.7%, +0.8%) |
| 0.4 | (63.5) | coarse M2 | 64 TPI (-0.8%) |
| 0.43 | (59.1) | BA 8BA | |
| 0.45 | (56.4) | coarse M2.2, M2.5 | 56 TPI (+0.8%) |
| 0.454 | 56 | UNC #2; UNF #3; cycle spokes | 0.45 mm (-0.8%) |
| 0.48 | (52.9) | BA 7BA | |
| 0.5 | (50.8) | coarse M3 | |
| 0.529 | 48 | UNC #3; UNF #4 | 0.53 mm (+0.2%) |
| 0.53 | (47.9) | BA 6BA | 48 TPI (-0.2%) |
| 0.577 | 44 | UNF #5 | 0.59 mm, 0.6 mm (+2.2%, +3.9%) |
| 0.59 | (43.1) | BA 5BA | 44 TPI (-2.2%) |
| 0.6 | (42.3) | coarse M3.5 | 44 TPI (-3.8%) |
| 0.635 | 40 | UNC #4, #5; UNF #6; BSW 1/8"; ME 1/8" to 1/2" (10) | 0.66 mm (+3.9%) |
| 0.66 | (38.5) | BA 4BA | 40 TPI (-3.8%) |
| 0.7 | (36.3) | coarse M4 | 36 TPI (+0.8%) |
| 0.706 | 36 | UNF #8; RMS objective | 0.7 mm, 0.73 mm (-0.8%, +3.5%) |
| 0.73 | (34.8) | BA 3BA | 36 TPI (-3.3%) |
| 0.75 | (33.9) | coarse M4.5; fine M6x0.75, M8x0.75, M10x0.75 | |
| 0.794 | 32 | UNC #6, #8; UNF #10; UNEF #12 to 3/8" (4); BSF 3/16"; ME 1/4" to 1" (10); cycle valves | 0.8 mm, 0.81 mm (+0.8%, +2.0%) |
| 0.8 | (31.7) | coarse M5 | 32 TPI (-0.8%) |
| 0.81 | (31.4) | BA 2BA | 32 TPI (-2.0%) |
| 0.9 | (28.2) | BA 1BA | 28 TPI (+0.8%) |
| 0.907 | 28 | UNF #12, 1/4"; UNEF 7/16", 1/2"; BSF 7/32"; BSP/BSPT 1/8" | 0.9 mm (-0.8%) |
| 0.941 | 27 | NPT 1/16"; NPT/NPS 1/8" | |
| 0.977 | 26 | BSF 1/4", 9/32"; BSB 1/4" to 2" (14); cycle axles | 1 mm (+2.4%) |
| 1 | (25.4) | coarse M6, M7; fine M8x1, M10x1, M12x1; BA 0BA; cycle crank bolt, axles, hanger, BB | 26 TPI (-2.3%) |
| 1.058 | 24 | UNC #10, #12; UNF 5/16", 3/8"; UNEF 9/16", 5/8", 11/16"; BSW 3/16"; cycle headset, BB, freewheel | |
| 1.155 | 22 | BSF 5/16" | |
| 1.25 | (20.3) | coarse M8, M9; fine M10x1.25, M12x1.25 | 20 TPI (+1.6%) |
| 1.27 | 20 | UNC 1/4"; UNF 7/16", 1/2"; UNEF 3/4" to 1" (5); BSW 1/4"; BSF 3/8"; cycle pedals | 1.25 mm (-1.6%) |
| 1.337 | 19 | BSP/BSPT 1/4", 3/8" | |
| 1.411 | 18 | UNC 5/16"; UNF 9/16", 5/8"; UNEF 1.1/16" to 1.1/2" (5); BSW 5/16"; BSF 7/16"; NPT/NPS 1/4", 3/8" | |
| 1.5 | (16.9) | coarse M10, M11; fine M12x1.5 to M30x1.5 (8) | |
| 1.587 | 16 | UNC 3/8"; UNF 3/4"; BSW 3/8"; BSF 1/2", 9/16" | |
| 1.75 | (14.5) | coarse M12 | 14 TPI (+3.7%) |
| 1.814 | 14 | UNC 7/16"; UNF 7/8"; BSW 7/16"; BSF 5/8", 11/16"; BSP/BSPT 1/2", 3/4"; BSP 5/8", 7/8"; NPT/NPS 1/2", 3/4" | 1.75 mm (-3.5%) |
| 1.954 | 13 | UNC 1/2" | 2 mm (+2.4%) |
| 2 | (12.7) | coarse M14, M16; fine M24x2, M27x2, M30x2 | 13 TPI (-2.3%) |
| 2.117 | 12 | UNC 9/16"; UNF 1" to 1.1/2" (5); BSW 1/2", 9/16"; BSF 3/4" | |
| 2.209 | 11.1/2 | NPT/NPS 1" to 2" (4) | |
| 2.309 | 11 | UNC 5/8"; BSW 5/8"; BSF 7/8"; BSP/BSPT 1" to 2" (4) | |
| 2.5 | (10.2) | coarse M18, M20, M22 | 10 TPI (+1.6%) |
| 2.54 | 10 | UNC 3/4"; BSW 3/4"; BSF 1" | 2.5 mm (-1.6%) |
| 2.822 | 9 | UNC 7/8"; BSW 7/8"; BSF 1.1/8", 1.1/4" | |
| 3 | (8.5) | coarse M24, M27 | |
| 3.175 | 8 | UNC 1"; BSW 1"; BSF 1.1/2" | |
| 3.5 | (7.3) | coarse M30, M33 | 7 TPI (+3.7%) |
| 3.629 | 7 | UNC 1.1/8", 1.1/4"; BSW 1.1/8", 1.1/4" | 3.5 mm (-3.5%) |
| 4 | (6.3) | coarse M36, M39 | |
| 4.233 | 6 | UNC 1.3/8", 1.1/2"; BSW 1.1/2" | |
| 4.5 | (5.6) | coarse M42, M45 | |
| 5 | (5.1) | coarse M48, M52 | 5 TPI (+1.6%) |
| 5.08 | 5 | UNC 1.3/4"; BSW 1.3/4" | 5 mm (-1.6%) |
| 5.5 | (4.6) | coarse M56, M60 | 4.1/2 TPI (+2.6%) |
| 5.644 | 4.1/2 | UNC 2"; BSW 2" | 5.5 mm (-2.6%) |
| 6 | (4.2) | coarse M64 |
Sizes by measured diameter
Pipe threads: the diameter is the thread OD (BSPT / NPT at the gauge plane). Metric: coarse pitch first, fine pitches in brackets. Unified / British: TPI after the name. BA: pitch in mm.
| Dia mm | Inch | Metric | Unified | British | Pipe (thread OD) |
|---|---|---|---|---|---|
| 1 | 0.039 | M1 x 0.25 | |||
| 1.2 | 0.047 | M1.2 x 0.25 | |||
| 1.4 | 0.055 | M1.4 x 0.3 | |||
| 1.52 | 0.060 | #0 UNF 80 | |||
| 1.6 | 0.063 | M1.6 x 0.35 | |||
| 1.7 | 0.067 | 10BA (0.35 mm) | |||
| 1.8 | 0.071 | M1.8 x 0.35 | |||
| 1.85 | 0.073 | #1 UNC 64 / UNF 72 | |||
| 1.9 | 0.075 | 9BA (0.39 mm) | |||
| 2 | 0.079 | M2 x 0.4 | |||
| 2.2 | 0.087 | M2.2 x 0.45 | #2 UNC 56 / UNF 64 | 8BA (0.43 mm) | |
| 2.5 | 0.098 | M2.5 x 0.45 | #3 UNC 48 / UNF 56 | 7BA (0.48 mm) | |
| 2.8 | 0.110 | 6BA (0.53 mm) | |||
| 2.84 | 0.112 | #4 UNC 40 / UNF 48 | |||
| 3 | 0.118 | M3 x 0.5 | |||
| 3.17 | 0.125 | #5 UNC 40 / UNF 44 | 1/8" BSW 40, 5BA (0.59 mm) | ||
| 3.5 | 0.138 | M3.5 x 0.6 | #6 UNC 32 / UNF 40 | ||
| 3.6 | 0.142 | 4BA (0.66 mm) | |||
| 4 | 0.157 | M4 x 0.7 | |||
| 4.1 | 0.161 | 3BA (0.73 mm) | |||
| 4.17 | 0.164 | #8 UNC 32 / UNF 36 | |||
| 4.5 | 0.177 | M4.5 x 0.75 | |||
| 4.7 | 0.185 | 2BA (0.81 mm) | |||
| 4.76 | 0.187 | 3/16" BSW 24 / BSF 32 | |||
| 4.83 | 0.190 | #10 UNC 24 / UNF 32 | |||
| 5 | 0.197 | M5 x 0.8 | |||
| 5.3 | 0.209 | 1BA (0.9 mm) | |||
| 5.49 | 0.216 | #12 UNC 24 / UNF 28 / UNEF 32 | |||
| 5.56 | 0.219 | 7/32" BSF 28 | |||
| 6 | 0.236 | M6 x 1 (fine 0.75) | 0BA (1 mm) | ||
| 6.35 | 0.250 | 1/4" UNC 20 / UNF 28 / UNEF 32 | 1/4" BSW 20 / BSF 26 | ||
| 7 | 0.276 | M7 x 1 | |||
| 7.14 | 0.281 | 9/32" BSF 26 | |||
| 7.94 | 0.312 | 5/16" UNC 18 / UNF 24 / UNEF 32 | 5/16" BSW 18 / BSF 22 | 1/16" NPT 27 | |
| 8 | 0.315 | M8 x 1.25 (fine 1, 0.75) | |||
| 9 | 0.354 | M9 x 1.25 | |||
| 9.52 | 0.375 | 3/8" UNC 16 / UNF 24 / UNEF 32 | 3/8" BSW 16 / BSF 20 | ||
| 9.73 | 0.383 | 1/8" BSP/BSPT 28 | |||
| 10 | 0.394 | M10 x 1.5 (fine 1.25, 1, 0.75) | |||
| 10.29 | 0.405 | 1/8" NPT/NPS 27 | |||
| 11 | 0.433 | M11 x 1.5 | |||
| 11.11 | 0.438 | 7/16" UNC 14 / UNF 20 / UNEF 28 | 7/16" BSW 14 / BSF 18 | ||
| 12 | 0.472 | M12 x 1.75 (fine 1.5, 1.25, 1) | |||
| 12.7 | 0.500 | 1/2" UNC 13 / UNF 20 / UNEF 28 | 1/2" BSW 12 / BSF 16 | ||
| 13.16 | 0.518 | 1/4" BSP/BSPT 19 | |||
| 13.72 | 0.540 | 1/4" NPT/NPS 18 | |||
| 14 | 0.551 | M14 x 2 (fine 1.5) | |||
| 14.29 | 0.562 | 9/16" UNC 12 / UNF 18 / UNEF 24 | 9/16" BSW 12 / BSF 16 | ||
| 15.88 | 0.625 | 5/8" UNC 11 / UNF 18 / UNEF 24 | 5/8" BSW 11 / BSF 14 | ||
| 16 | 0.630 | M16 x 2 (fine 1.5) | |||
| 16.66 | 0.656 | 3/8" BSP/BSPT 19 | |||
| 17.14 | 0.675 | 3/8" NPT/NPS 18 | |||
| 17.46 | 0.688 | 11/16" UNEF 24 | 11/16" BSF 14 | ||
| 18 | 0.709 | M18 x 2.5 (fine 1.5) | |||
| 19.05 | 0.750 | 3/4" UNC 10 / UNF 16 / UNEF 20 | 3/4" BSW 10 / BSF 12 | ||
| 20 | 0.787 | M20 x 2.5 (fine 1.5) | |||
| 20.64 | 0.812 | 13/16" UNEF 20 | |||
| 20.95 | 0.825 | 1/2" BSP/BSPT 14 | |||
| 21.34 | 0.840 | 1/2" NPT/NPS 14 | |||
| 22 | 0.866 | M22 x 2.5 (fine 1.5) | |||
| 22.22 | 0.875 | 7/8" UNC 9 / UNF 14 / UNEF 20 | 7/8" BSW 9 / BSF 11 | ||
| 22.91 | 0.902 | 5/8" BSP 14 | |||
| 23.81 | 0.938 | 15/16" UNEF 20 | |||
| 24 | 0.945 | M24 x 3 (fine 2, 1.5) | |||
| 25.4 | 1.000 | 1" UNC 8 / UNF 12 / UNEF 20 | 1" BSW 8 / BSF 10 | ||
| 26.44 | 1.041 | 3/4" BSP/BSPT 14 | |||
| 26.67 | 1.050 | 3/4" NPT/NPS 14 | |||
| 27 | 1.063 | M27 x 3 (fine 2) | 1.1/16" UNEF 18 | ||
| 28.57 | 1.125 | 1.1/8" UNC 7 / UNF 12 / UNEF 18 | 1.1/8" BSW 7 / BSF 9 | ||
| 30 | 1.181 | M30 x 3.5 (fine 2, 1.5) | |||
| 30.2 | 1.189 | 7/8" BSP 14 | |||
| 31.75 | 1.250 | 1.1/4" UNC 7 / UNF 12 / UNEF 18 | 1.1/4" BSW 7 / BSF 9 | ||
| 33 | 1.299 | M33 x 3.5 | |||
| 33.25 | 1.309 | 1" BSP/BSPT 11 | |||
| 33.4 | 1.315 | 1" NPT/NPS 11.1/2 | |||
| 34.92 | 1.375 | 1.3/8" UNC 6 / UNF 12 / UNEF 18 | |||
| 36 | 1.417 | M36 x 4 | |||
| 38.1 | 1.500 | 1.1/2" UNC 6 / UNF 12 / UNEF 18 | 1.1/2" BSW 6 / BSF 8 | ||
| 39 | 1.535 | M39 x 4 | |||
| 41.91 | 1.650 | 1.1/4" BSP/BSPT 11 | |||
| 42 | 1.654 | M42 x 4.5 | |||
| 42.16 | 1.660 | 1.1/4" NPT/NPS 11.1/2 | |||
| 44.45 | 1.750 | 1.3/4" UNC 5 | 1.3/4" BSW 5 | ||
| 45 | 1.772 | M45 x 4.5 | |||
| 47.8 | 1.882 | 1.1/2" BSP/BSPT 11 | |||
| 48 | 1.890 | M48 x 5 | |||
| 48.26 | 1.900 | 1.1/2" NPT/NPS 11.1/2 | |||
| 50.8 | 2.000 | 2" UNC 4.1/2 | 2" BSW 4.1/2 | ||
| 52 | 2.047 | M52 x 5 | |||
| 56 | 2.205 | M56 x 5.5 | |||
| 59.61 | 2.347 | 2" BSP/BSPT 11 | |||
| 60 | 2.362 | M60 x 5.5 | |||
| 60.32 | 2.375 | 2" NPT/NPS 11.1/2 | |||
| 64 | 2.520 | M64 x 6 |
Odd and special threads
| Series | Form | TPI | Sizes | Where you find it |
|---|---|---|---|---|
| ME 40 | 55° Whitworth | 40 | 1/8" to 1/2" | Model engineering - small fittings, glands, unions |
| ME 32 | 55° Whitworth | 32 | 1/4" to 1" | Live steam - boiler fittings, injectors, unions |
| BSB (brass) | 55° Whitworth | 26 | 1/4" to 2" | Thin-wall brass tube, lamp and gas fittings, brassware |
| BSC / CEI (cycle) | 60° | 26 (24, 20) | 1/8" to 1.3/8" | Old British bicycles and motorcycles - see the cycle table |
| UNEF | 60° Unified | 32 to 18 | #12 to 1.1/2" | Aircraft, instruments, thin-wall parts - sizes in the table below |
| RMS (microscope) | 55° Whitworth | 36 | 0.800" (20.32 mm) | Microscope objectives |
| Camera / tripod | 60° UNC | 20 / 16 | 1/4" and 3/8" | Camera mounts, tripods, light stands |
Cycle threads
| Cycle part | Thread | Note |
|---|---|---|
| Bottom bracket - English (BSA) | 1.370" x 24 TPI | Drive side left-hand |
| Bottom bracket - Italian | 36 mm x 24 TPI | Both sides right-hand |
| Bottom bracket - French (old) | 35 x 1 mm | Both sides right-hand |
| Freewheel / screw-on hub | 1.370" x 24 TPI | Same as English BB |
| Threaded headset (1") | 1" x 24 TPI | |
| Pedal - 3-piece crank | 9/16" x 20 TPI | Left pedal left-hand |
| Pedal - 1-piece crank / kids | 1/2" x 20 TPI | Left pedal left-hand |
| Axle - front, solid | 5/16" x 26 TPI | BSC / CEI |
| Axle - rear, solid | 3/8" x 26 TPI | BSC / CEI |
| Axle - front, quick release | 9 x 1 mm | Hollow axle |
| Axle - rear, quick release | 10 x 1 mm | Hollow axle |
| Crank bolt (square taper) | M8 x 1 | |
| Derailleur hanger | 10 x 1 mm | |
| Spoke (14 G / 2 mm) | 2.2 mm x 56 TPI | Rolled thread |
| Valve - Presta | 5.2 mm x 32 TPI | 5V1 |
| Valve - Schrader (car type) | 7.7 mm x 32 TPI | 8V1 |
Lead-screw threads
| Trapezoidal (DIN 103) | Major mm | Pitch mm |
|---|---|---|
| Tr8 x 1.5 | 8 | 1.5 |
| Tr10 x 2 | 10 | 2 |
| Tr12 x 3 | 12 | 3 |
| Tr14 x 3 | 14 | 3 |
| Tr16 x 4 | 16 | 4 |
| Tr18 x 4 | 18 | 4 |
| Tr20 x 4 | 20 | 4 |
| Tr22 x 5 | 22 | 5 |
| Tr24 x 5 | 24 | 5 |
| Tr26 x 5 | 26 | 5 |
| Tr28 x 5 | 28 | 5 |
| Tr30 x 6 | 30 | 6 |
| Tr32 x 6 | 32 | 6 |
| Tr36 x 6 | 36 | 6 |
| Tr40 x 7 | 40 | 7 |
| Tr44 x 7 | 44 | 7 |
| Tr50 x 8 | 50 | 8 |
| Tr60 x 9 | 60 | 9 |
| Acme (ASME B1.5) | Major mm | TPI |
|---|---|---|
| 1/4"-16 | 6.35 | 16 |
| 5/16"-14 | 7.94 | 14 |
| 3/8"-12 | 9.53 | 12 |
| 7/16"-12 | 11.11 | 12 |
| 1/2"-10 | 12.70 | 10 |
| 5/8"-8 | 15.88 | 8 |
| 3/4"-6 | 19.05 | 6 |
| 7/8"-6 | 22.23 | 6 |
| 1"-5 | 25.40 | 5 |
| 1.1/8"-5 | 28.58 | 5 |
| 1.1/4"-5 | 31.75 | 5 |
| 1.3/8"-4 | 34.93 | 4 |
| 1.1/2"-4 | 38.10 | 4 |
| 1.3/4"-4 | 44.45 | 4 |
| 2"-4 | 50.80 | 4 |