Thread standards explained: metric, BSW, UNC and Acme
Metric, Whitworth, Unified, Acme, a plain-English tour of the thread standards you will meet on studs and fasteners, and when each is used.
Threads look interchangeable until they aren’t. Mixing standards is a classic cause of parts that won’t mate. Here is a quick reference.
Metric (M)
The global default. Written as M-diameter × pitch, e.g. M16 × 2.0. Coarse pitches for general use, fine pitches where you need more preload or finer adjustment.
BSW / Whitworth
The classic British inch standard with a 55° thread angle, still common on older machinery and in parts of Indian industry. Written in threads-per-inch (TPI).
UNC / UNF
The Unified inch standard (60° angle), UNC coarse, UNF fine. Common where American-spec equipment is involved.
Acme & square
Not fastening threads but power threads, for lead screws, jacks and clamps that move load. Their trapezoidal (Acme) or square profile carries axial force far better than a V-thread. Our coarse 5-TPI studs are a good example.
Why it matters for quoting
Always give the full callout and, if you can, the mating part’s standard. We cut and roll all of the above, but only if we know which one you need.
Have a part that fits this?
Tell us the material and spec, we’ll quote it.
Keep reading
MS vs SS316: choosing the right steel for your turned part
Mild steel or stainless? The right choice depends on corrosion, strength, cost and finish. A practical guide for buyers speccing machined components.
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The six things to include on a drawing or enquiry so a machine shop can quote quickly and accurately, without a dozen back-and-forth emails.
Have a drawing or a part to match?
Send us your specification, sample or worn part. We’ll reverse-engineer it, quote quickly and machine it to precision, from a single prototype to volume production.