TAPER PLUG AND RING GAUGES TO BS21 AND ISO7/2

 

Gauges for testing pipe threads where pressure tight joints are made on threads to :- BS 21: 1985 and ISO 7/2 1982

 

BS 21 specifies requirements of pipe threads for joints made pressure tight by the mating of threads, and are taper external, and 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.

 

The pipe fittings range in size from 1/16 to 6.0

 

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 threads.

 

System B gauges are intended for use where there may be no other adequate method of production control.

 

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

 

When gauging taper external threads with the 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, 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 hand tight 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/Not 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. The size range for threads is 1/8 to 2.0. These threads are designated Rs (1/4)

 

Go/ Not Go parallel screw plugs can be to test parallel internal threads.

 

The manufacturing limits for these parallel gauges will be specified in BS 919: Pt 2: 1971.

 

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 designed to carry out the same gauging function, there are differences in design, and differences in the gauge manufacturing limits.