Thread tolerances: 6g, 6H, 4g6g, 2A and coating allowances

The tolerance class of a thread defines how far the pitch diameter and the major or minor diameter may deviate from the nominal size. Under ISO 965, it consists of a number for the tolerance grade and a letter for the tolerance position: lower-case letters for external threads, capital letters for internal threads. 6g for the bolt and 6H for the nut are the standard classes for medium fits; they apply whenever nothing else is stated on a drawing.

The most important classes

• 6g / 6H: Standard for bolts and nuts; the g stands for a small fundamental deviation, meaning a little clearance in the thread.
• 4g6g / 5H: Tighter tolerance for fits with little clearance, for example on fine threads of adjusting components.
• 6e, 6f: Larger fundamental deviation, as an allowance for coatings such as zinc flake or thick electroplated layers.
• 6az / 6AX to ISO 10684: For hot-dip galvanised assemblies; the nut is tapped oversize so that there is room for the thick zinc layer on the bolt.
• 2A / 2B and 3A / 3B to ASME B1.1: The American equivalents for UN threads; 2A is standard, 3A is a close fit, and the suffix 2A mod. permits a coating allowance.

Why the coating must be taken into account

Every coating adds thickness: a thread that sits exactly in the middle of the tolerance before coating may no longer accept the GO gauge afterwards. RS Randack deliberately rolls threads that are to be coated so that they are within gauge after coating, and checks them again with gauges when they return from the coater. With hot-dip galvanising, the nut is instead tapped oversize to ISO 10684.

How testing is carried out

With GO and NOT GO ring gauges, the thread micrometer for the pitch diameter and, for small threads, on the profile projector; gauges are calibrated and traceable. For large threads, the pitch diameter is documented and shown in the inspection report on request.

What you should specify

Tolerance class, coating and coating thickness, whether the nut is to be supplied as well, and whether the thread must be within gauge before or after coating.