The highest Unified thread class, specified for socket head cap screws, aerospace hardware and other parts where a close, repeatable fit matters. A 3A external thread has zero allowance and a narrower tolerance band than 2A, so it costs more to make and leaves no room for plating unless the thread is sized before coating. A 3A screw assembles with a 2B hole, but the fit is only as good as the looser part.
These plated class 3A screws won't go in the 3B holes, the zinc ate the whole tolerance, they needed pre-plate sizing.
Also heard as
- 3A
- 3B
Related terms
Thread class
A number-and-letter grade, such as 2A or 3B, that sets how much tolerance and allowance an inch thread has.
SHCS
Socket head cap screw: a round-headed, high-strength screw driven with a hex key into a tapped hole.
Zinc plating
An electroplated zinc layer, usually with a clear, blue or yellow passivate, the cheapest common fastener finish.
Pitch diameter
The diameter at which thread thickness equals the gap between threads, the dimension that controls fit.