AN Fitting Thread Guide: JIC, NPT, BSP and ORB Explained

The most common cause of leaks in a DIY plumbing build isn't poor assembly technique — it's mixing thread standards. AN, NPT, BSP, and ORB threads can look similar, partially engage with each other, and feel tight when tightened. But if the seal mechanism doesn't match, the fitting will leak. Sometimes immediately, sometimes after heat cycling a few times.

This guide covers the four thread standards you'll encounter in motorsport plumbing, how to identify each one, and which adapter you need to connect them correctly.

The Four Thread Standards at a Glance

The seal mechanism is what matters — not just the thread pitch or diameter. Two fittings can share a similar thread size but seal in completely different ways.

AN/JIC — The Motorsport Standard

AN (Army-Navy) fittings use a 37° flare seat — a precision-machined cone that mates metal-to-metal with the female fitting. No sealant, no tape, no gasket. The flare does all the work.

• Straight (non-tapered) thread
• Reusable — can be assembled and disassembled repeatedly without losing seal integrity
• Leak-free when assembled correctly to the correct torque
• Used throughout AN plumbing: hose ends, bulkheads, adapters, tees

How to identify: look inside the female fitting — you'll see a 37° internal cone (flare). The male fitting has a matching 37° external nose.

For dash size and application guidance, see our AN Fitting Size Guide.

AN03 male-to-male adapter in stainless steel — the 37° flare nose is visible on both ends. No sealant required.

NPT — National Pipe Taper

NPT is a US standard where the taper of the thread itself creates the seal as the fitting is tightened. The threads deform slightly as they engage, which is why NPT always requires PTFE tape or thread sealant — without it, the metal-to-metal contact alone will leak.

• Threads visibly taper — they get narrower toward the end of the fitting
• Always use PTFE tape or anaerobic thread sealant — never dry
• Common on: US-spec engine blocks, oil pans, thermostat housings, fuel pressure regulators, some pumps
• Thread sizes: 1/8 NPT, 1/4 NPT, 3/8 NPT, 1/2 NPT are most common in motorsport

How to identify: run a straight edge along the threads — NPT threads taper visibly. The fitting will also feel like it tightens progressively as it's wound in, rather than stopping at a defined point.

Critical warning: NPT threads will partially engage with AN threads. They can feel tight and look connected — but the 37° flare seat won't form correctly and the fitting will leak. Never use NPT fittings in AN ports without the correct adapter.

AN8 to 1/8 NPT port adapter — this is the correct way to connect an AN8 line to a 1/8 NPT port. The NPT end seals into the port with PTFE tape; the AN end accepts a standard hose end fitting.

BSP — British Standard Pipe

BSP is the European and Japanese equivalent of NPT, but with different thread geometry. There are two variants:

• BSPP (parallel): straight threads that seal against a flat face using a bonded seal (dowty washer) or copper washer — common on European and Japanese engine blocks, gearboxes, and power steering pumps
• BSPT (tapered): tapered threads similar to NPT, but with a different thread pitch — requires PTFE tape or sealant

How to identify BSPP: parallel threads (no taper) with a flat seating face on the port. The fitting winds in to a defined stop rather than tightening progressively.

How to identify BSPT: tapered threads, similar appearance to NPT — measure thread pitch to distinguish (BSP uses metric pitch; NPT uses imperial).

Important: BSPP and NPT have similar thread counts in some sizes but different pitches and seat angles — they are not interchangeable. A 1/4 BSP fitting will partially engage a 1/4 NPT port and feel tight, but will leak.

M10x1 to AN3 stainless steel adapter — used to connect AN3 brake or clutch lines to metric M10x1 ports common on European and Japanese vehicles.

ORB — O-Ring Boss

ORB (O-Ring Boss) uses straight threads and an O-ring that seats against a flat machined boss on the port face. The O-ring does all the sealing — no flare, no taper, no sealant required.

• Straight thread — winds in to a defined stop
• O-ring groove visible on the male fitting, just behind the thread
• Very reliable seal — not sensitive to torque variation like tapered threads
• Common on: modern engine blocks, hydraulic components, some AN-to-block adapters

How to identify: look for a groove on the male fitting just behind the threads — this holds the O-ring. The port will have a flat machined face for the O-ring to seat against.

Note: ORB threads are straight and can look similar to AN threads, but the seal mechanism is completely different. An ORB fitting in an AN port (or vice versa) will not seal correctly.

AN8 to -10 ORB straight adapter — the ORB end seals via O-ring against a flat boss; the AN8 end accepts a standard hose end fitting with a 37° flare.

How to Identify Thread Type Without a Thread Gauge

You don't need specialist tools to identify most thread types — a visual check and a pair of calipers will get you there in most cases:

1. Is it tapered or parallel? Hold a straight edge along the threads. Taper = NPT or BSPT. Parallel = AN, BSPP, or ORB.
2. Is there a 37° flare inside the female fitting? If yes → AN/JIC.
3. Is there an O-ring groove on the male fitting? If yes → ORB.
4. Is there a flat seating face on the port with parallel threads? If yes → BSPP (use a bonded seal).
5. Tapered thread with no flare? Measure thread pitch — imperial pitch = NPT, metric pitch = BSPT.

Common Thread Adapter Combinations

AN6 to M14x1.5 power steering adapter — connects AN6 braided hose to the metric port on a power steering pump or rack.

AN6 to 1/8 NPT port adapter in blue anodised — the NPT end threads into the port with PTFE tape; the AN6 end accepts a standard hose end.

The Golden Rule — Never Mix Thread Standards

• Never use PTFE tape on AN fittings — it prevents the 37° flare from seating correctly and will cause leaks
• Never force a fitting that doesn't wind in smoothly — cross-threading damages the seat and the fitting is scrap
• Always identify the port thread before ordering — a thread gauge set costs less than a seized engine or a brake failure
• When in doubt, use an adapter — the correct adapter costs a few euros and eliminates all risk

Need the Right Adapter?

Identify your port thread, match it to the table above, and order the correct adapter. All Maxflo Motorsport thread adapters are available in stainless steel and anodised aluminium — built to the same AN/JIC standard as the rest of our range.

→ Shop Maxflo Motorsport

Also in this series:

• How to Size AN Fittings: A Complete Guide
• PTFE vs Stainless Steel Braided Hose — Which Should You Choose?