Taper Plug and Ring Gauges
to BS21 and ISO7/2

Gauges for testing pipe threads where pressure tight joints are made on the threads to BS 21: 1985 and ISO 7/2 2008.

BS21 specifies requirements of pipe threads for joints made pressure tight by the mating of threads, and are taper external, taper internal, parallel, external and parallel internal threads.

The thread form is based on the Whitworth profile. The taper is 1:16 on diameter, normal to the axis.

There are two systems for gauging pipe threads specified in this standard.

System A gauges are intended for use where there are additional production control methods to ensure accuracy of the threads. System B gauges are intended for use where there may be no other adequate method of production control.

System A comprises two types of gauge. A taper full form screw ring, and a taper full form screw plug.

When gauging taper external threads with the taper full form screw ring, the small end of the product thread should lie within the plus and minus tolerance of the small end of the ring.

When gauging taper or parallel internal threads with the taper full form plug, the end of the product thread should lie within the plus and minus tolerance from the gauge plane step of the plug.

System B comprises four types of gauge. A taper full form screw ring, a taper full form screw plug, a taper plain ring and a taper plain plug.

When gauging taper external threads with the taper full form ring or with the taper plain ring, the small end of the product thread should lie between the faces, or flush with either face of the step on the gauge.

When gauging taper or parallel internal threads with the taper full form screw plug or with the taper plain plug, the end face of the product should lie between the faces, or flush with either face of the step on the gauge.

When using any of the above gauges, assembly should be handtight only, taking care not to use excessive force

Designations

External taper threads and taper ring gauges are designated R (1/4)

Internal taper threads and taper plug gauges are designated Rc (1/4)

Internal parallel threads and parallel plug gauges are designated Rp (1/4)

Go/No go parallel screw rings can be supplied to test special parallel external threads for gas appliances where pressure tight seals are made on machined faces. The size range for threads is 1/8 to 2.0. These threads are designated Rs (1/4)

Go/No go parallel screw plugs can be supplied to test parallel internal threads.

The manufacturing limits for these parallel gauges will be as specified in BS 919: Pt. 2: 2007.

ISO 7/2 specifies one gauging system only for testing pipe threads.

The standard specifies three types of gauge. A full form taper screw ring, a full form taper screw plug, both used for testing the product thread, and a modified form taper screw check plug for testing the ring during manufacture and for testing wear following use.

Whilst the gauges in this standard are manufactured to the same basic product diameters as the BS 21 pipe thread, and are designated to carry out the same gauging function, there are differences in design and differences in the gauge manufacturing limits.