§1The unified system
The Unified Thread Standard is the inch-based thread system of North America — the same 60° thread form as metric, but sized in inches and counted in threads per inch rather than measured as a pitch.
Agreed between the United States, Britain and Canada after the Second World War to replace three incompatible national systems, the unified thread shares the metric thread’s 60° flank angle and general form (the threads section covers the geometry), so the two look alike on a drawing. They differ in how they are specified. Metric states the pitch — the millimetres from one thread to the next. The unified system states the count — how many threads fall in an inch — which is the reciprocal, and it is the source of the initially odd-looking designations of §2. It survives wherever American practice does: aerospace, automotive, plant and machinery across North America, and legacy equipment worldwide. The rest is the same fastening physics of the torque-and-tension page, in different units.
Contents§2Reading a designation
A unified thread is written as a chain of four items — diameter, threads per inch, series, class — and once parsed it is fully specified.
Take the designation apart (the hero). 1/4 is the nominal major diameter in inches. 20 is the threads per inch — so the pitch is its reciprocal, 1 ÷ 20 = 0.050 in, which is 1.27 mm. UNC is the series: Unified National Coarse. 2A is the class of fit, the letter A meaning an external thread (a bolt); B would be internal (a nut) — see §5. Below ¼ inch, diameters are given as gauge numbers instead of fractions, so a #10-24 is a numbered size (the machine screws page derives those from d = 0.060 + 0.013 N inches). The one habit to build is the reciprocal: threads per inch and pitch are inverses, so a bigger TPI number means a finer, more closely spaced thread — the opposite of the metric intuition, where a bigger number means coarser.
§3Coarse against fine
Every nominal size comes in a coarse series and a fine series — UNC and UNF — and the choice is a real engineering decision, not a formality.
UNC (coarse) has fewer, deeper threads: it assembles fast, tolerates dirt, damage and a rough tapped hole, strips less readily in soft materials because its threads are deeper, and is the default for general work. UNF (fine) has more, shallower threads: it has a larger stress area and so is stronger in tension (§4), it resists vibration loosening a little better because its helix is shallower, and its finer pitch allows more precise adjustment — a given turn of the nut advances it less. The trade is that UNF is slower to run down, easier to cross-thread, and its shallow threads strip more readily in soft metal. In short: coarse for general assembly and soft materials, fine for strength, precision and thin-walled parts. The same coarse/fine split, for the same reasons, appears in the metric system.
Contents§4The stress area
A threaded bolt does not carry load on its full nominal diameter — the threads cut into it — so its strength is reckoned on a tensile stress area computed from the diameter and the thread count.
The formula takes an effective diameter between the thread’s root and pitch line, so a shallower thread leaves more metal. For 1/4-20 UNC: As = 0.7854 × (0.25 − 0.9743/20)² = 0.0318 in². For 1/4-28 UNF, with its shallower thread: As = 0.7854 × (0.25 − 0.9743/28)² = 0.0364 in² — 14.3% more area in the identical nominal size. That is the whole quantitative case for fine threads: same bolt diameter, a seventh more tensile capacity, purely because less metal was cut away. It also shows why the nominal size alone never tells you a bolt’s strength — the thread series and the grade (§6) both have to be known before any load can be worked out.
§5Classes of fit
The class number states how tightly the bolt and nut threads fit — the thread equivalent of the tolerance-and-fit system of the dimensioning section.
The letter says which member: A for an external thread (bolt, screw), B for an internal one (nut, tapped hole). The number says how close the tolerance: 1A/1B is a loose fit, generous with tolerance, for quick assembly and where dirt, plating or damage must be tolerated; 2A/2B is the general-purpose standard, the class the vast majority of commercial fasteners are made to and the one to assume if none is stated; 3A/3B is a close fit, tightly toleranced, for precision and safety-critical work where thread engagement must be exact. As with the shaft-and-hole fits, the two members are specified independently and can be mixed — a 3A bolt in a 2B nut, say. The practical rule is that 2A/2B is the default and covers ordinary engineering; reach for 3A/3B only where precision genuinely earns its extra cost, and 1A/1B where speed and tolerance of muck matter more than snugness.
Contents§6Grades and head markings
An inch bolt’s strength is set by its SAE grade, and — usefully — the grade is stamped on the head as radial lines you can count.
The convention is simple and readable in the field (the hero): a plain unmarked head is grade 2, low-carbon and weak; three radial lines mean grade 5, a medium-carbon quenched-and-tempered bolt with a proof strength of 85 000 lbf/in²; six lines mean grade 8, alloy steel at 120 000 lbf/in². The count is not decorative — “lines plus two” gives the grade. Working the loads for a 1/4-20 (As = 0.0318 in²): a grade 5 has a proof load of 0.0318 × 85 000 = 2705 lbf (12.0 kN), while a grade 8 carries 0.0318 × 120 000 = 3819 lbf (17.0 kN) — 1.41 times as much from an identical-looking bolt. This is why substituting a hardware-store grade 2 for a specified grade 8 is a genuine hazard, and why counting the lines before fitting a bolt is a habit worth having. The metric system does the same job with its property-class numbers.
§7Quick reference
The working core of the page on one card rack.
Designation
1/4-20 UNC 2A
Ø · TPI · series · fit
Pitch
p = 1 / TPI
20 TPI → 0.050 in = 1.27 mm
Stress area
As = 0.7854(D − 0.9743/n)²
UNF has 14.3% more than UNC
Fits
A external · B internal
2A/2B is the default
Grades
lines + 2 = grade
3 → gr5 · 6 → gr8 (1.41×)