Thread Dimensions

Sources: National Bureau of Standards Handbook H28 (1969), Screw-Thread Standards for Federal Services, Part I, Section 2 (Unified threads), with FED-STD-H28/2B (1991), its notices and its Appendix A hole size tables. IS 14962 (Part 1):2001 and (Part 2):2001, ISO General Purpose Metric Screw Threads — Tolerances, the same as ISO 965-1 and ISO 965-2.

Basic sizes: pitch P (1 ÷ threads per inch); basic pitch diameter = D − 0.649519 P; basic minor diameter of the nut = D − 1.082532 P; minor diameter of the screw at the root = D − 1.226869 P.

Unified tolerances: pitch diameter tolerance = C × (0.0015 ∛D + 0.0015 √Le + 0.015 ∛P²), with C = 1.5, 1.0 and 0.75 for 1A, 2A and 3A and 1.95, 1.3 and 0.975 for 1B, 2B and 3B. Le, the length of engagement, is D for UNC, UNF and the 4, 6 and 8 UN series, and 9 P for UNEF and the 12 to 32 UN series, whose D term is the series' representative diameter. Allowance for 1A and 2A = 0.3 × (the same sum); 3A has none, and no nut class has one. Major diameter tolerance of the screw = 0.090 ∛P² for 1A, 0.060 ∛P² for 2A and 3A. Minor diameter tolerance of the nut: 0.25 P − 0.4 P² for 1B and 2B from 1/4 in up (0.15 P coarser than 4 threads per inch); below 1/4 in and for 3B, 0.05 ∛P² + 0.03 P ÷ D − 0.002, held between the handbook's upper limit of 0.394 P and its lower limit.

Metric tolerances: the fundamental deviation es (screw) or EI (nut) for the class's letter, and the tolerances Td, Td2, TD1 and TD2 for its grade, from the standard's tables for the pitch and the diameter band. Screw: major d + es − Td to d + es, pitch d₂ + es − Td2 to d₂ + es, minor (reference) d₃ + es. Nut: minor D₁ + EI to D₁ + EI + TD1, pitch D₂ + EI to D₂ + EI + TD2, major D + EI minimum.

Checked against the printed tables: every limit in H28 Table 2.21 (8,787 values) and all 480 in ISO 965-2. Where this page differs from the 1969 H28 print, the print is wrong: 12 misprints, each shown by the row’s own arithmetic (for example, 3.625-6 UN 2B pitch diameter printed 4.5293, larger than the thread, for 3.5293), and 3 minor diameters one unit off in the last place, which FED-STD-H28/2B (1991) prints as given here. The 5/8-11 UNC allowance is the printed 0.0016 (the formula gives 0.0017), as published class 2A charts carry it. For 13/16-16 and 7/8-16 UN class 2A, H28 prints a pitch diameter tolerance of 0.0049; the formula gives 0.0048, which is used here.

Fit: on the pitch diameter, the nut's minimum less the screw's maximum (the tightest) to the nut's maximum less the screw's minimum (the loosest).