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Technical Specifications

BA and G Series Thread Gauges

BA Series

Gauges for testing threads having a full form BA profile are manufactured to BS 93: 2008 and BS 919: Part 2: 2007.

BS 93 specifies basic information for the British Association thread form, and tabulates diameters for threads from 0BA to 25BA. However, 10BA is generally the smallest size manufactured as stock items. (Smaller sizes may be quoted for on application.)

Basic Form BA Thread
  • Basic Form BA Thread
  • H = 1.136 34 x p
  • h = 0.600 00 x p
  • r = 0.180 83 x p
  • s = 0.268 17 x p

External threads from 0BA to 10BA are provided with an allowance of 0.001” on diameter to allow for easy assembly with the internal thread.

Provision is made for two classes of fit for external threads; Close and Normal for sizes from 0BA to 10BA inclusive and, for Normal class, only for sizes from 11BA to 16BA. Close class is intended for precision parts where there is controlled accuracy of form and pitch. Normal class is intended for general commercial use.

There is only one class of fit for internal threads, which is not designated.

BS 919: Pt 2: 2007 specifies the limits for gauges and are similar in style to those used for Whitworth threads.

G Series

Gauges for testing threads to ISO228:2003 (G Series).

ISO228 specifies details of pipe threads where pressure tight joints are not required and also specifies the limits for gauges for testing the product thread.

ISO 228/1 - 2003 for the designation, dimensions and tolerances of the product thread and to ISO 228/2 – 2003, for verification by means of limit gauges.

The standard specifies basic information for Whitworth form fastening pipe threads and tabulates diameters having a thread size designation in the range1/16 to 6 inclusive.

There is no allowance provided for external or internal threads.

Provision is made for two classes of fit for external threads: Class A and Class B, and one class only for the internal thread, which is not designated. The external class A tolerance is equivalent to the internal thread tolerance. The class B tolerance is twice that of the class A tolerance.

The specification states that, where no class is specified, class B should be assumed.

Internal and external threads are designated by the letter G followed by the thread size designation. This designation refers to the pipe size and not to the thread diameter.

The gauge limits and principles are similar to the limits specified in ISO 1502.