Gauges for testing parallel threads having a full form Whitworth profile are manufactured to BS 84: 2007 and BS 919: Part 2: 2007.
BS 84 specifies basic information for the Whitworth form, and tabulates diameters for threads designated BSW and BSF.
BSW is a coarse thread series from 1/8" to 6" diameter.
BSF is a fine thread series from 3/16" to 3" diameter.
Provision is made for three classes of fit for external threads. Close, Medium and Free and two classes for internal threads; Medium and Normal.
Close class applies to threads requiring a snug fit and is only used where there is a controlled accuracy of form and pitch. Medium class applies to better class interchangeable threads. Free and Normal classes apply to the majority of commercial quality threads.
Unless otherwise specified by the customer, gauges for all internal and external product threads are made to the basic sizes as specified in BS 84, with the exception of Medium and Free fit bolts up to and including 3/4” diameter. These have a plating allowance, the magnitude of which is dependant on diameter and pitch.
BS919: Pt 2: 2007 specifies the limits for gauges with threads having a full form profile.
The gauge limits are dependant upon the diameter/pitch. Generally, the larger the diameter, the larger the limit within defined bands.
The types of gauge which are recommended to be used are similar to those used for ISO and Unified threads, with the exception that the Go plugs and Go rings test the major, effective and minor diameters of the product threads (Maximum material limit). The No Go gauges test the effective diameter only.