Guide · Couplers, adapters and swivels
NPT vs BSP vs M22 pressure washer threads
Check taper, diameter and pitch in that order. How NPT, BSPT, BSPP and M22 differ, how each seals, and when thread sealant belongs on a fitting.
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Short answer
Identify a thread in order: is it tapered or straight, what is its diameter, and what is its pitch. That is how Swagelok's five-step method starts. NPT is tapered with a 60-degree angle; BSPT is tapered and BSPP parallel, both at 55 degrees; M22 is a straight metric thread with a 1.5 mm pitch that seals on an O-ring, not on the thread.
Swagelok's five steps
Swagelok's thread identification procedure has five steps: decide if the thread is tapered or straight, measure the diameter, find the pitch, determine the thread standard, then identify the end connection.[2] Swagelok also warns that even experienced workers sometimes find threads hard to identify, and that its tables are identification aids, not inspection criteria.
Step 1: tapered or parallel
Dixon explains that a thread whose diameter stays the same along its length is parallel, while a tapered thread gets smaller.[3] To check, Dixon suggests measuring the diameter with calipers at both the start and the end of the threads.[3]
- Tapered: NPT and BSPT. NPT is tapered with a 60-degree thread angle; BSPT has the same kind of taper at 55 degrees.[2]
- Parallel (straight): BSPP, NPSM and metric threads such as M22.
Step 2 and 3: diameter and pitch
Dixon says to measure a male thread's outside diameter with a caliper or O.D. tape, and that non-metric pitch is expressed as threads per inch.[3] Swagelok defines pitch as threads per inch for fractional and pipe threads, and as the distance between adjacent threads for metric ones.[2]
Two pitch facts do most of the work:
- 1/4 and 3/8 in NPT have 18 threads per inch; 1/4 and 3/8 in BSPT have 19.[2]
- M22 x 1.5 has a 22 mm nominal size and a 1.5 mm pitch.[2] Westinghouse labels its 14 mm adapter "M22x1.5 - 14mm".[4]
Dixon notes that measured values may differ slightly from its chart values.[3] It gives no tolerance figure, so treat a near-miss as a reason to check the maker's document, not as a match.
Step 4: the standard
Some pairs look alike:
- NPT vs BSPT: both tapered, but 60 versus 55 degrees. Swagelok says the 1/2 and 3/4 in sizes can be hard to tell apart and may need an optical comparator.[2]
- BSPP vs NPSM: both straight. Swagelok defines BSPP as 55 degrees and NPSM as 60 degrees.[2]
The US standard for general-purpose inch pipe threads, including NPT and NPSM, is ASME B1.20.1.[5]
| Thread | Shape | Angle | Pitch at 1/4 and 3/8 in | Seals on |
|---|---|---|---|---|
| NPT | Tapered | 60° | 18 TPI | The thread, with sealant |
| BSPT | Tapered | 55° | 19 TPI | The thread, with sealant |
| BSPP | Parallel | 55° | Not in our sources | Gasket or O-ring |
| M22 x 1.5 | Parallel | Metric | 1.5 mm | O-ring on the stem |
How each one seals
Swagelok says tapered pipe threads seal on the threads and always need a sealant, while straight threads seal with a gasket, O-ring or metal contact, so sealant is not required or recommended on them.[2] On an M22 joint the seal is the O-ring on the stem, not the thread.
If an O-ring joint leaks, tape is not the fix. Champion's manual says never to repair leaking connections with sealant, and to replace damaged O-rings instead.[1]
One maker exception: Westinghouse's PWEXT manual tells users to put thread seal tape on that kit's inlet fitting threads and includes the tape.[4] Follow that for that kit only.
Reading thread names
Dixon explains that "1/2-14 NPT" means a pipe thread of nominal 1/2 in inside diameter with 14 threads per inch, while the M in a metric name gives the thread's outer diameter in millimeters.[3] So "1/2 in" NPT is not 1/2 in across the thread, but "M22" is 22 mm across.
FPT means a female (internal) pipe thread and MPT a male one. Dixon also says female NPT is not available on hose swivel nuts.[3]
Where pipe threads show up
Pipe threads appear on guns and fittings sold by size and port thread. Suttner lists most of its spray guns with a 3/8 in FPT inlet and a 1/4 in FPT outlet.[6] A consumer M22 hose does not screw into those ports; it needs an adapter named for both ends. See the adapter guide.
Once you know your ends
Picks that fit2 products
If both ends you identified are M22 with a 14 mm stem, choose the Flexzilla HFZPW3425M (1/4 in x 25 ft, 3,100 PSI at 70°F).
More on choosing
If both are 3/8 in quick-connect, choose the SIMPSON 41028 (3/8 in x 50 ft, 4,500 PSI, cold water). Either hose's rating must meet or exceed your machine's maximum. On Amazon, check that 'Ships from / Sold by' shows Amazon.com or the brand.
M22 hose with a stated 14 mm stem
Flexzilla HFZPW3425M pressure washer hose, 1/4 in x 25 ft, M22 (14 mm stem)
3/8 in quick-connect hose for cold water
SIMPSON Monster Hose 41028 3/8 in x 50 ft 4,500 PSI cold-water pressure washer hose, 3/8 in QC
Flexzilla states the 14 mm stem and 3,100 PSI rating at 70°F;[7] SIMPSON states the 41028's 3/8 in plug and socket and 4,500 PSI rating.[8]
Next
Questions
Is M22 the same as 3/8 NPT?
No. M22 x 1.5 is a straight metric thread with a 1.5 mm pitch that seals on an O-ring. 3/8 NPT is a tapered inch pipe thread with 18 threads per inch that seals on the thread with sealant.[2]
How do I tell NPT from BSP?
NPT has a 60-degree thread angle; BSPT and BSPP have 55 degrees. At 1/4 and 3/8 in, NPT has 18 threads per inch and BSPT 19. Swagelok warns the 1/2 and 3/4 in sizes can be hard to tell apart without an optical comparator.[2]
Should I use Teflon tape on pressure washer fittings?
Only on tapered pipe threads, which seal on the thread. Straight threads such as M22 seal on a gasket or O-ring, where Swagelok says sealant is not required or recommended; follow a maker's own kit instructions where they differ. A leak at an O-ring joint means a damaged O-ring, which Champion says to replace.[2][1]
What does 3/8 FPT mean?
A female (internal) tapered pipe thread of 3/8 in nominal size. Suttner lists most of its guns with a 3/8 in FPT inlet and a 1/4 in FPT outlet.[6]