Thread Sizes & Fittings — Identification Guide
This guide covers common mechanical and hydraulic thread standards and fittings used in automotive and industrial applications. It explains identification, dimensional notes, sealing method, common uses, and tips for assembly.
NPT (National Pipe Taper)
NPT is a U.S. tapered pipe thread standard (ANSI/ASME B1.20.1). NPT threads seal by interference and thread deformation — use thread sealant or tape for gas-tight joints.
- Identification: Tapered threads, measured by nominal pipe size (e.g., 1/8", 1/4", 1/2").
- Sealing: Thread sealant or PTFE tape.
- Common uses: Plumbing, air and fuel lines, adapters.
BSP (British Standard Pipe) — BSPT / BSPP
BSP comes in tapered (BSPT) and parallel (BSPP) forms. BSPT seals similarly to NPT (tapered), while BSPP often relies on an O-ring or washer for sealing when used with a mating port.
AN / JIC / Flare (37° and 45°)
AN (Army-Navy) fittings use a 37° flare seat (often called JIC in hydraulic contexts). Metric flare and SAE flares may use a 45° seat (common on brake lines and automotive tubing).
- Identification: Flared tube end with matching flare nut and male fitting.
- Sealing: Metal-to-metal flare face (37° or 45°) — avoid over-tightening; use proper flare tools.
- Common uses: High-pressure hydraulics, fuel lines (AN), brake lines (45° SAE).
ORB (O-Ring Boss)
ORB fittings use a straight thread with a recessed port containing an O-ring for sealing. Threads themselves do not seal — the O-ring does.
- Identification: Parallel threads with a hex shoulder and a visible O-ring in the female port.
- Sealing: Elastomeric O-ring; ensure O-ring material compatibility.
- Common uses: Hydraulic ports on pumps, valves, and manifolds.
Metric Threads (ISO)
Metric threads follow the ISO metric standard (M# × pitch). Use appropriate thread gauges or measure major diameter and pitch to identify.
Compression Fittings
Compression fittings use a ferrule (or two) to compress against tubing. Common in instrumentation and low-pressure hydraulic lines.
Identification Cheat Sheet
- Measure outer diameter (major) and pitch (threads per inch or mm pitch).
- Check thread form: tapered vs parallel.
- Inspect sealing method: thread deformation, O-ring, flare face, or ferrule.
- Match with common standards: NPT, BSPT, BSPP, AN/JIC, ORB, Metric.
Assembly Best Practices
- Clean threads and mating surfaces.
- Use proper sealants where required (PTFE tape for NPT, thread paste for high-pressure gas).
- Do not reuse damaged ferrules or flares; replace if deformed.
- Torque to manufacturer specs — avoid over-tightening.
Reference Tables
For precise dimensions refer to the relevant standards (ASME, ISO, SAE). Below are quick references to common sizes (nominal):
NPT nominal sizes (examples)
- 1/8" NPT — approx major OD 0.405"
- 1/4" NPT — approx major OD 0.540"
- 3/8" NPT — approx major OD 0.675"
- 1/2" NPT — approx major OD 0.840"
AN sizing
AN sizes are given as -2, -3, -4 etc. where the number corresponds to 1/16ths of an inch of O.D. tubing (e.g., AN-4 = 4/16 = 1/4" tube).