NPT vs BSPP vs JIC Thread Identification Guide | Apex Flow

Mixing thread standards is one of the costliest mistakes in fluid power and piping. NPT, BSPP, and JIC threads can look nearly identical to the eye, yet forcing the wrong pairing together cross-threads the fitting, destroys the seal, and can fail under pressure. This guide gives you the measurable differences — taper, thread angle, pitch, and seat geometry — plus a step-by-step method to identify any unknown thread with a caliper and a thread pitch gauge.

Apex Flow Solutions supplies NPT brass and pipe fittings along with the adapters that bridge thread standards. Use the reference tables below to identify what you have and what you need.

Need a thread adapter?

NPT-to-JIC, NPT-to-BSPP, and other crossover adapters let you connect mismatched threads safely. Contact our team with your two thread types and we will spec the right adapter.

The Three Thread Families at a Glance

NPT (National Pipe Tapered) is the dominant North American pipe thread. It has a 60° thread angle and a 1°47' taper per side, and it seals on the threads themselves — which is why it requires Teflon tape or pipe dope. BSPP (British Standard Pipe Parallel, G) has a 55° thread angle and is parallel (straight); it seals with a bonded washer or O-ring at the face, not on the threads. JIC (Joint Industry Council, SAE J514 37°) is a straight UNF thread that seals on a 37° flare cone — the threads only provide clamping force, and the metal-to-metal cone makes the seal.

Side-by-side comparison of NPT tapered, BSPP parallel, and JIC flare thread profiles

NPT threads taper and seal on the threads; BSPP threads are parallel and seal at a washer/O-ring; JIC threads are parallel and seal on a 37° flare cone.

Tapered vs Parallel: The First Check

The fastest first cut is taper. Lay a straightedge against the thread crests. If the diameter visibly narrows toward the end of the thread, it is tapered — almost certainly NPT (or BSPT). If the crests are parallel for the full thread length, it is a straight thread — BSPP or JIC. This one observation eliminates half the possibilities before you ever pick up a gauge. The second tell is the seat: a 37° machined cone at the fitting nose points to JIC; a flat sealing face for a washer or an O-ring groove points to BSPP.

NPT vs BSPP vs JIC Comparison Table

Attribute	NPT	BSPP (G)	JIC (37°)
Thread form	Tapered	Parallel	Parallel (UNF)
Thread angle	60°	55°	60°
Seal method	On threads (needs sealant)	Bonded washer / O-ring	37° flare cone (metal-to-metal)
Sealant required?	Yes	No (washer/O-ring)	No
Standard	ASME B1.20.1	ISO 228-1	SAE J514
Typical use	N. American pipe/plumbing	European/Asian hydraulics	Hydraulics, fuel, instrumentation

Thread OD & Pitch Identification Chart

This is where NPT and BSPP are most often confused — at small sizes their major diameters are close, but their pitch (TPI) differs. Measure the male thread major diameter with a caliper and the pitch with a thread gauge, then match both columns.

Nominal Size	NPT Male OD / TPI	BSPP Male OD / TPI
1/8"	0.405" / 27	0.383" / 28
1/4"	0.540" / 18	0.518" / 19
3/8"	0.675" / 18	0.656" / 19
1/2"	0.840" / 14	0.825" / 14
3/4"	1.050" / 14	1.041" / 14
1"	1.315" / 11.5	1.309" / 11

JIC sizes are not pipe-nominal — they are referenced by tube OD and a UNF thread, e.g. a -6 JIC is 9/16"-18 UNF for 3/8" tube, and a -8 JIC is 3/4"-16 UNF for 1/2" tube. If you measure a straight thread with a 37° nose cone, identify it by the JIC dash size rather than a pipe size.

Thread pitch gauge and caliper being used to identify a fitting thread

Two tools settle nearly every thread ID: a caliper for major diameter and a thread pitch gauge for TPI. Match both to the chart.

Step-by-Step Thread Identification

Work through these steps in order and you will identify almost any thread in under a minute:

Step 1 — Taper check: Is the thread tapered or parallel? Tapered narrows toward the tip (likely NPT/BSPT). Parallel stays constant (BSPP or JIC).
Step 2 — Seat check: Look at the sealing face. A 37° machined cone means JIC. A flat face for a washer or an O-ring groove means BSPP. No seat, sealing on threads, means NPT.
Step 3 — Measure major diameter: Use a caliper on the male thread crests (or the female thread root for a port). Record the value.
Step 4 — Measure pitch: Use a thread pitch gauge to count TPI. NPT 1/4" is 18 TPI; BSPP 1/4" is 19 TPI — that one-thread difference is the deciding factor at small sizes.
Step 5 — Cross-reference: Match diameter + pitch + seat against the tables above to confirm the standard and size.

Standards & References

NPT is defined by ASME B1.20.1; the dryseal variant NPTF (used for fuel and hydraulics without sealant) is ASME B1.20.3. BSPP (designation G) and the tapered British thread BSPT (designation R) are defined by ISO 228-1 and ISO 7-1 respectively, with a 55° Whitworth thread form. JIC 37° flare fittings follow SAE J514 and the equivalent ISO 8434-2. Knowing the controlling standard also tells you the correct assembly torque and sealing method.

Frequently Asked Questions

How can I tell NPT from BSPP threads?

NPT is tapered and seals on the threads; BSPP is parallel and seals with a washer or O-ring. If you cannot see the taper, measure pitch: at 1/4" NPT is 18 TPI while BSPP is 19 TPI. The thread angle also differs (60° NPT vs 55° BSPP).

Will NPT and BSPP fittings connect together?

They should not be mixed. The differing taper and 60°/55° thread angles prevent a reliable seal and can cross-thread the parts. Use a dedicated NPT-to-BSPP adapter instead.

What does JIC stand for and how do I identify it?

JIC stands for Joint Industry Council. It is a straight 37° flare fitting per SAE J514. Identify it by the parallel UNF thread combined with a 37° machined cone at the sealing nose — the cone makes the seal, not the threads.

Do JIC and AN fittings interchange?

JIC (SAE J514) and AN (military 37° flare) share the 37° seat and thread sizes and will physically mate, but AN parts are manufactured to tighter tolerances and material specs. For critical aerospace or racing work, do not substitute JIC for AN.