The following table defines recommended industry standard sizes for External ACME General Purpose Threads per American National Standard ASME/ANSI B1.5.

There are three classes of General Purpose ACME threads, 2G, 3G, and 4G each class provides some clearance on all standard size diameters for free movement, and are used in assemblies with the internal thread rigidly fixed and movement of the external thread in a direction perpendicular to its axis limited by its bearing or bearings.

Designs should only utilize external and internal ACME threads of the same class and type, with Class 2G being the preferred choice. If less backlash or end play is desired, Classes 3G and 4G are provided. Class 5G is not recommended for new designs.