A&B Thread Pitch
Q: What is the difference between an A and B thread pitch?
A: An A Thread class or pitch signifies the outer threads, therefore an A applies to a bolts, screws, threaded rod and studs. Whereas a B thread class applies to the internal threading. B Thread class is for nuts or internal threads where the threads are inside the fastener.
Thread Classes 1, 2 & 3
Q: What are the different thread pitch classes?
A: There are also 3 classes that denote the thread tolerance for the pitch. For bolts and other fasteners where the threads are external the classes are 1A, 2A and 3A. In contrast, for nuts where the threads are internal, it would be 1B, 2B and 3B.
Class 1A or 1B thread pitch is for the loosest tolerance. They provide the largest amount of play or clearance in assembly and are best used when easy/rapid assembly and disassembly of bolts is required.
Class 2A or 2B is the most common thread class. Class 2 provides a combination of interchangeability, quality and fit.
Class 3A and 3B are the highest tolerance thread class. These are mostly used in precise applications where tolerances are most critical and a tight fit is required.