A designation of fit for Unified inch threads: classes 1A, 2A and 3A apply to external threads and 1B, 2B and 3B to internal ones, with higher numbers meaning tighter tolerances. Classes 1A and 2A include an allowance, a built-in clearance that leaves room for plating and easy assembly, while 3A has none. Class 2A with 2B is the general commercial fit used on the great majority of bolts and nuts.
Thread class on the manifold ports is 3B, so use the 3B plug gauges, the shop 2B set won't tell you a thing.
With no class on the callout, inch hardware is assumed 2A and 2B; ask for 3A only when the fit needs it, because it costs more.
Also heard as
- class of fit
Related terms
Class 3A/3B
The tightest standard inch thread fit, with no allowance on the external thread, used for precision hardware.
Pitch diameter
The diameter at which thread thickness equals the gap between threads, the dimension that controls fit.
Thread plug gauge
A hardened GO and NO GO gauge for internal threads: the GO end must screw in, the NO GO end must not.
ISO metric thread
The 60-degree metric screw thread, written as M plus diameter and pitch in millimeters, such as M10 x 1.5.