Thread Types Explained: UNC, UNF, NPT, BSP, Metric, and ACME

Why So Many Thread Standards?

Because different applications need different threads. Fastening, sealing, power transmission, and motion control all use different thread forms. Here’s the decoder ring.

Unified National (UN) — Standard Fastener Threads

UNC (Coarse)

The default thread for bolts, nuts, and screws in the US. Designated like: 1/4-20 UNC (1/4″ diameter, 20 threads per inch).

• Faster assembly (fewer turns)
• More tolerant of slightly damaged threads
• Better for softer materials (aluminum, plastic) — less likely to strip
• Use UNC unless you have a reason not to

UNF (Fine)

Same diameters, more threads per inch: 1/4-28 UNF.

• Stronger in tension (more thread engagement per inch)
• Better for thin-wall applications
• Finer adjustment capability
• More susceptible to cross-threading and galling
• Common in automotive and aerospace

Metric (ISO) Threads

Designated by diameter × pitch: M10×1.5 (10mm diameter, 1.5mm pitch). Standard pitch is coarse; fine pitch is specified explicitly (M10×1.25).

• Dominant worldwide (everything except legacy US equipment)
• Class 4.8, 8.8, 10.9, 12.9 correspond to increasing strength
• Class 8.8 metric ≈ Grade 5 US; Class 10.9 ≈ Grade 8

Pipe Threads

NPT (National Pipe Taper)

Tapered threads that seal by interference — the threads themselves create the seal when tightened. Always use thread sealant (Teflon tape or pipe dope).

• Used for: Plumbing, pneumatics, hydraulic fittings, compressed air
• Sizes: 1/8″ to 24″ — but the number is NOMINAL, not actual diameter
• 1/8 NPT actual OD: ~0.405″

NPTF (Dryseal)

Modified NPT with tighter tolerances designed to seal without tape or dope. Theoretically. In practice, most people still use sealant.

NPS / NPSM (Straight Pipe)

Same thread form as NPT but not tapered. Does NOT seal by itself — needs an O-ring or gasket. Used where you need a mechanical connection, not a pressure seal.

BSP (British Standard Pipe)

Used outside North America. BSPT (tapered) seals like NPT. BSPP (parallel) seals with a washer or O-ring. BSP and NPT are NOT interchangeable — the thread angle is different (55° vs 60°).

Power / Motion Threads

ACME Thread

Trapezoidal 29° thread form designed for power transmission: lead screws, vises, clamps, jacks. Strong, low friction, easy to manufacture.

Buttress Thread

Asymmetric — high load capacity in one direction. Used in jack screws, artillery breeches, and pipe couplings under axial load.

Ball Screw

Not really a thread — recirculating ball bearings ride in a helical groove. Near-zero friction. Used in CNC machines, robotics, and linear actuators. Expensive but precise.

Quick Reference

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