Q: My customer called out a 3a thread for their hex head screws. Is this different from a 2a thread pitch?
A: Yes these are 3 different classes of fit for external inch series threads for studs, bolts, and screws 1A, 2A and 3A. 2A thread class is a medium fit, whereas a 3A thread class is a tight fit and 1A is a loose fit (rarely used though). The thread fit will determine the tightness or looseness of the mating threads when an externally threaded fastener is screwed into an internally threaded nut. 2A is the most common because of cost, performance, and they are easily manufactured. 3A is used for applications with close tolerances where more strength is required. 3A is more frequently used for socket head cap screws and socket set screws while 2A is used more for studs, bolts, and screws. Also as a point of reference, the “A” stands for the external male thread whereas a “B” would be for an internal female thread like a nut.