AN vs ORB vs NPT vs NPSM: The No-Leak Guide (Sealant Rules Inside)
Leaks are optional. Matching thread and sealing surface is mandatory. Use this guide to identify ports at a glance, apply sealant only when it actually helps, and choose adapters that fit the first time—especially on transmission cases.
Quick Decision Chart
- ORB: Straight thread with an O-ring counterbore; seals at the O-ring. No sealant on threads.
- AN (37° flare): Male/female cone seats; seals at the flare. Never apply sealant to flare faces.
- NPT (tapered pipe): Tapered threads; seals on the threads. Use fuel-rated PTFE paste or tape, 2–3 wraps starting 1–2 threads back.
- NPSM (straight pipe for trans cases): Male adapter threads into the case; threads do not seal. Sealing occurs at the case seat or with the included washer. No thread sealant.
Build smarter: Start with ethanol-safe PTFE hose, choose the right ends, then adapt once.
Hose Bundles PTFE Hose Ends AN Adapters Install Tools
Know Your Sealing
AN — 37° flare; seal at the seat
AN uses a 37° metal-to-metal cone. The seal is the seat, not the threads. Keep flare faces clean and dry. Transition from factory hardline with a proper hardline-to-AN adapter.
ORB — Straight thread; seal at the O-ring
ORB retains with straight threads and seals on a captured O-ring, so you can clock the fitting and snug until the O-ring just compresses. Choose FKM/Viton for ethanol/E85.
NPT — Tapered pipe; seal on the threads
NPT tightens as it goes. Use fuel-rated PTFE paste or tape. Wrap clockwise (facing the male threads) and start 1–2 threads back to keep debris out of the system.
NPSM — Straight pipe for transmission cases
NPSM is a straight pipe thread used in many GM and Ford transmission case cooler ports. In our world, you install a male NPSM adapter that converts the case to AN—threads do not seal. Sealing occurs at the case seat or via the included sealing washer. No thread sealant on the threads.
- Common applications (1/4–18 NPSM case ports): GM TH350, TH400, 700R4, 200-4R, 4L60E, 4L70E, 4L80E, 4L85E; Ford C5, AOD, AODE, 4R70W, E4OD/4R100.
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4L70E/4L80E/4L85E note: Same 1/4-18 NPSM threads; many 4L80E/4L85E units require a long/nipple rear-port fitting. We stock both styles:
- Standard pair (no nipple): 1/4-18 NPSM → AN (pair)
- Long/nipple style: 1/4-18 NPSM → AN (long/nipple)
- Washer included: Our adapters include a sealing washer—clean the boss, install dry, and torque to seal.
- Do not substitute NPT: Forcing NPT into a straight NPSM case port can crack the housing or never seal correctly.
GM vs. Ford Port Depth — choosing the right NPSM adapter
- Standard-depth case ports (most GM + many Ford): Use our 1/4-18 NPSM → AN pair for TH350, TH400, 700R4, 200-4R, 4L60E, plus Ford C5/AOD/AODE/4R70W and others.
- Deep/stepped rear port (common on 4L80E/4L85E; some 4L70E): Use the long/nipple NPSM → AN adapter at the rear port (front usually standard). This ensures the nose reaches the internal seat and clears the passage.
- Quick self-check before install: Thread the adapter in by hand. If the hex bottoms before the washer contacts the boss—or if the short nose doesn’t reach the seat—swap to the long/nipple style for that port.
Bottom line: standard pair covers most builds; 4L80E/4L85E rear ports often need the long/nipple adapter for proper sealing and flow.
Washer & Torque Guidance — NPSM adapters
- Prep the boss: Clean the case boss/seat; remove paint, thread tape, or old washer residue. The threads don’t seal on NPSM—your seal is the seat/washer.
- Install the included washer: Our adapters ship with a sealing washer. Seat the washer squarely against the boss.
- Snug, then set: Thread the adapter in by hand until the washer touches. Tighten gradually to compress the washer. As a rule of thumb, aim for snug + 1/8 turn, then leak-check after the first heat cycle.
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Washer types behave differently:
- Nylon/composite: compresses with modest effort; stop when fully seated—don’t crush to deformation.
- Aluminum/copper (crush style): requires firmer load to form; once seated, small angle changes make big force increases—creep up slowly.
- Never use paste/tape on NPSM threads. Over-torque risks boss damage with zero sealing benefit. If a leak persists, inspect the seat/washer and verify adapter length (standard vs. long).
Torque values vary with washer material and surface condition; tighten to achieve washer compression and a dry seal, not a target number.
Size & Thread Reference
AN / JIC 37° (male thread reference)
| AN Size | Male Thread (UNF) | Common use |
|---|---|---|
| AN-6 | 9/16-18 | EFI feed/return, filters, rails |
| AN-8 | 3/4-16 | High-HP feed, dual pumps, trans coolers |
| AN-10 | 7/8-14 | Very high flow/pump inlets |
ORB (O-ring boss — seals at O-ring)
| ORB Size | Thread (UNF) | Common use |
|---|---|---|
| -4 ORB | 7/16-20 | Compact regs, small sensor/manifold ports |
| -6 ORB | 9/16-18 | Fuel rails, compact filters, regulators |
| -8 ORB | 3/4-16 | Filters, pump inlets/outlets |
| -10 ORB | 7/8-14 | High-flow filters, cooler adapters, surge tanks |
| -12 ORB | 1-1/16-12 | Remote filters, dry-sump/large cooler circuits |
NPT (tapered pipe — seals on threads)
| NPT Size | TPI | Common use |
|---|---|---|
| 1/8" NPT | 27 | Pressure/temp sensors, small ports |
| 1/4" NPT | 18 | Legacy pump/regulator ports, vents |
| 3/8" NPT | 18 | Fuel cell vents, larger gauge/sender adapters |
| 1/2" NPT | 14 | Cooler adapters, pump housings, breathers |
| 5/8" NPT | — | If labeled “5/8,” confirm spec; often straight or UNF (e.g., 5/8-18) rather than NPT |
| 3/4" NPT | 14 | Remote filter mounts, big cooler circuits |
NPSM (straight pipe — trans case ports)
| Size | Type | Notes |
|---|---|---|
| 1/4-18 NPSM (male) | Straight pipe (mechanical) | Threads do not seal; seal at case seat or with washer. GM TH350/TH400/700R4/200-4R/4L60E/4L70E/4L80E/4L85E; Ford C5/AOD/AODE/4R70W/E4OD/4R100. Long/nipple style often needed on 4L80E rear port. |
Install Tips
- PTFE hose: wrap the cut area with tape before cutting to control fray; deburr cleanly.
- EFI filtration: use ≤10 μm on the pressure side to protect injectors.
- Stainless threads on stainless: a tiny touch of anti-seize on threads is fine—never on flare faces.
- NPSM: clean the case boss, use the included washer, and torque to seal. Do not add paste/tape to the threads.
More help: PTFE Hose & Fittings Install Instructions | AN Sizes & What They Mean | AN Hose 101 Hub
LS-Swap Quick Picks
Troubleshooting
- AN flare weep: Inspect cone faces for nicks, contamination, or 45° vs 37° mix-up. Clean and re-seat; snug, don’t crush.
- ORB seep: Replace or lightly oil the O-ring; tighten until just compressed.
- NPT persistent leak: Reseat with fresh paste/tape (2–3 wraps). Avoid bottoming out. Consider adapting to AN.
- NPSM case drip: Inspect the washer and case seat surface; threads do not seal. Replace washer if damaged and verify adapter length (standard vs. long).
Build it once, enjoy it forever.
Ethanol-safe PTFE hose, correct AN ends, and purpose-built adapters.
Hose Bundles PTFE Hose Ends Adapters Trans Case → AN (Pair) Long/Nipple Version
FAQ
Are transmission case cooler ports NPT?
Do I need thread sealant on NPSM adapters?
Which transmissions use 1/4-18 NPSM?
What’s the difference between NPT and NPSM?
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