In other words, an H1 limit is one . 0005 increment larger than the minimum limit or Go gage, an H2 is two . 0005 increments (or . 001) larger than the minimum limit, an H3 is three .
Classes 2A-2BClass 2A external threads have an allowance, Class 2B internal threads do not. Classes 2A and 2B, for most engineering applications, offer the optimum thread fit that balances fastener performance, manufacturing convenience and economy.
?Class 2B is the typical fit for commercial products and is used for fasteners and threaded parts. Class 3B is the tightest tolerance and used for close fit and high strength fastening applications where a close and snug fit are required.
The Class of a thread indicates the tolerance allowed in manufacturing. Class 1 is a loose fit, Class 2 is a free fit, Class 3 is a medium fit, and Class 4 is a close fit. Aircraft bolts are almost always manufactured in the Class 3, medium fit.