How to Measure and Identify Threads
Here at Merlin, we get asked how to measure threads, such as how a 1/2”BSP thread is measured and how it compares to a UNF thread and so on. So here, we hope to offer some guidance on how to measure and identify threads.
BSP Threads (British Standard Pipe)
BSP threads are very common in motorsport engine plumbing and pipework. All BSP threads have a 60 degree taper on the end of the male thread. Whilst BSP female fittings have a 60 degree convex seat which seals on the 60 degree taper of a male thread. BSP threads are identified by their nominal size, but this is where confusion can arise. For example, a ½” BSP male thread does not measure ½” in diameter. The following table should explain this better:-
| Nominal BSP Thread Size | Male Thread Actual Outside Diameter | Female Thread Actual Inside Diameter |
| 1/8” BSP (by 28 TPI) | 3/8” (9.5mm) | 11/32” (8.7mm) |
| ¼” BSP (by 19 TPI) | 33/64” (13.1mm) | 15/32” (11.9mm) |
| 3/8” BSP (by 19 TPI) | 21/32” (16.6mm) | 19/32” (15.1mm) |
| ½” BSP (by 14 TPI) | 13/16” (20.6mm) | ¾” (19.05mm) |
| 5/8” BSP (by 14 TPI) | 29/32” (23.0mm) | 13/16” (20.6mm) |
| ¾” BSP (by 14 TPI) | 1 1/64” (25.8mm ) | 31/32” (24.6mm) |
*TPI = Threads per inch
A tapered version of BSP is also available, known as BSPT. The dimensions of the threads are the same as BSP.
JIC (AN) SAE 37 Threads
SAE 37° thread, which are also commonly known as JIC or AN (Air Force/Navy) thread is an American thread and is almost universally used for performance and motorsport plumbing. These types of fittings use UNF threads, these UNF threads are identified by the O.D. of the male thread. The following table explains:-
| Dash Size | Thread Name | Male Thread Actual Outside Diameter | Female Thread Actual Inside Diameter |
| -2 | 5/16” UNF (by 24 TPI) | 5/16” (7.93mm) | 9/32” (7.14mm) |
| -3 | 3/8” UNF (by 24 TPI) | 3/8” (9.52mm) | 11/32” (8.73mm) |
| -4 | 7/16” UNF (by 20 TPI) | 7/16” (11.11mm) | 3/8” (9.52mm) |
| -5 | ½” UNF (by 20 TPI) | ½” (12.70mm) | 7/16” (11.11mm) |
| -6 | 9/16” UNF (by 18 TPI) | 9/16” (14.28mm) | ½” (12.70mm) |
| -8 | ¾” UNF (by 16 TPI) | ¾” (19.05mm) | 11/16” (17.46mm) |
| -10 | 7/8” UNF (by 14 TPI) | 7/8” (22.22mm) | 13/16” (20.63mm) |
| -12 | 1 1/16” UNF (by 12 TPI) | 1 1/16” (26.98mm) | 31/32” (24.60mm) |
| -16 | 1 5/16” UNF (by 12 TPI) | 1 5/16” (33.33mm) | 1 7/32” (30.95mm) |
*TPI = Threads per inch
Metric Threads
Metric threads are becoming more popular in motorsport and performance plumbing. Available with both convex and concave seats and some varying thread pitches. Identified by the O.D. of a male thread. A female metric fitting I.D. should measure 1.5mm less than the O.D. of a male fitting. The following table explains this:-
| Metric Thread Size | Male Thread Actual Outside Diameter | Female Thread Actual Outside Diameter |
| M10 | 10mm | 8.5mm |
| M12 | 12mm | 10.5mm |
| M14 | 14mm | 12.5mm |
| M16 | 16mm | 14.5mm |
| M18 | 18mm | 16.5mm |
| M22 | 22mm | 20.5mm |
| M26 | 26mm | 24.5mm |
| M30 | 30mm | 28.5mm |
The above threads are the more popular threads we see in use, there are many more metric threads available. There are also many differing thread pitches in use. The pitch is determined by measuring the distance from the top of one thread to the top of the next.
NPT Threads
NPT threads (National Pipe Taper) (also sometimes known as NPTF for fuel) is popular in America and is seen in oil systems and more popular in fuel lines, particularly in Facet fuel pumps. A tapered thread which is designed to seal as a male tapered thread is screwed in to a female tapered port/fitting helping to illiminate leaks. Often it is advisable to use PTFE tape or thread sealant with NPT fittings. Identified by their nominal sizes, but this is where confusion can arise. For example, a 1/8” NPT male thread does not measure 1/8” in diameter. The following table should help to explain this better:-
| Nominal NPT Thread Size | Male Thread Actual O.D. | Female Thread Actual I.D. |
| 1/8” NPT (by 27 TPI) | 13/32” (10.3mm) | 6/16” (9.4mm) |
| ¼” NPT (by 18 TPI) | 35/64” (13.7mm) | 8/16” (12.4mm) |
| 3/8” NPT (by 18 TPI) | 11/16” (17.3mm) | 10/16” (15.7mm) |
| ½” NPT (by 14 TPI) | 27/32” (21.3mm) | 12/16” (19.3mm) |
| 5/8” NPT (by 14 TPI) | 29/32 (23mm) | 13/16” (21.1mm) |
| ¾” NPT (by 14 TPI) | 1 1/16” (26.9mm) | 1” (24.9mm) |
| 1” NPT (by 1 ½ TPI) | 1 5/16” (33.3mm) | 1 4/16” (31.5mm) |
* TPI = Threads per inch
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