Nut threads have no allowance: at its minimum a Class 2B or 3B thread is at basic size, so there is no room in it for a coating on the bolt. Class 2B is the default for ASTM A194 and A563 nuts; 3B holds a closer tolerance where the drawing calls for it.
Hot-dip zinc is far too thick to fit, so ASTM A563 has nuts for galvanized bolts tapped oversize after galvanizing, by a set allowance: 0.018 in. on a 1/2 in.-13 nut, 0.024 in. on 1 in.-8, 0.050 in. from 1-3/4 in. up. The bolt is threaded to the normal Class 2A before it is galvanized, and the zinc on its thread protects the bare thread of the nut. Overtapped nuts are proof load tested after galvanizing and tapping.
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model dataNut drawn to the ASME B18.2.2 heavy hex proportions for 1 in.-8 (1.575–1.625 in. across flats, 0.956–1.012 in. thick), bored to the Class 2B minor diameter from ASME B1.1 (0.865–0.890 in.); nut threads gauged to ASME B1.2, as ASTM A962 requires. Overtapping after hot-dip galvanizing, the 0.024 in. allowance and proof load testing after overtapping from ASTM A563 (§7.4.1, Table 5, §6.2.2); bolts for overtapped nuts threaded Class 2A before galvanizing (ASTM A307, A449). Galvanizing is done outside, by a Houston processor. The spiral-point tap, machine, fixture, gauges, tote and bolt are generic. Turning speeds are illustrative and marked as slow motion or time-lapse. In the cutaways the tap and bolt are sectioned with the nut; the overtap is drawn true to scale, and the zinc is drawn much thicker than it is.
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