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

Need threaded parts manufactured? Send us your specs — we’ll make sure the threads are right.