Thread fit is a measure of the tightness or looseness between a male and the female corresponding female thread. Classes of thread fit are specific combinations of tolerances and allowances that are applied to male and female threads.
From a plating point of view, tolerances/allowances applying to fasteners are important because they provide a measure of the maximum thickness of corrosion resistant coatings that can used in the fastener assembly.
The tolerances/allowances (class) for external Unified thread are designated by the letter ‘A’ preceded by a number. Ex; Class 1A, 2A and 3A.
Internal Unified threads are designated by letter ‘B’ preceded by a number. Ex: class 1B, 2B and 3B. Therefore it follows that class 1A bolts fits a class 1B nut, class 2A with 2B and class 3A with 3B.
In the metric system –according to ISO 965, two tolerances classes of thread fits are recommended. 6g bolt with 6H nut and 6e bolt with 6G nut.
Relationship of pitch diameter, allowances and tolerances for classes of fit for 1/2-13 UNC thread.
This class is a replacement for the original American National Class 1 thread and ideally suited for work of rough commercial quality where quick and easy assembly and disassembly are prime requisites. As such, the 1A & 1B class is rarely specified for mechanical fastener.
This class is predominantly use for production of close to 90% of all commercial bolts, nuts, screws and similar products produced in the North America. It is by far the most popular thread class specified for inch series mechanical fasteners, mainly because it offers a thread fit that balance performance, manufacturing convenience, plating allowance and economy.
Used where closeness of fit and accuracy between mating parts for high quality work is a prime requisite. It is suited for high strength fasteners such as socket cap and set screws, aerospace bolts and nuts and other fasteners intended for service in applications where safety is of critical concern.
Unified INCH External Screw Thread Designation
Unified INCH Internal Screw Thread Designation