Air Gauging Explained

How Air Plugs Work

Air flowing through the plug from the gauge unit escapes to atmosphere through two or more jets recessed below the body diameter. When the plug is inserted in the bore, any variations in the clearance between each jet and and the work piece causes a difference in pressure which is indicated by the gauge unit. Two setting rings of known size are used to adjust the unit for magnification and datum zero.

Measuring Range

The maximum range is determined by the jet size. Plugs are normally made to read equally about the mean of the work tolerances although it is sometimes convenient to vary this practice as listed below:

Full Scale

Allows use of the full scale range of the gauge unit, jet size permitting.

Half Range

Allows use of half the scale on the minus side and all the scale on the plus side.


Plugs and their setting rings made to suit a specified work tolerance.


Allows the entire tolerance to be displayed on the plus side of the scale, thereby allowing early entry of the plug into the bore.

Special Gauge Applications

Using a special combination of jets, many features difficult to measure any other way can be gauged easily and rapidly.
The diagrams illustrate some of the more common applications.

The method for gauging Centre Distance makes allowances for any variation of the actual size of the two bores. A particularly useful application of combined jets is the simultaneous gauging of Centre Distance, and mutual parallelism in two planes (bow & twist) of the bores of engine connecting rods.

Measuring Head Identification

Measuring heads must only be used with the system for which they are manufactured. All heads i.e plugs and rings carry a symbol corresponding to the unit, as shown in the illustration below.
Clearline & Air Electronic Symbol

Features & Terminology

Open Jet Plugs

2-jet plugs for gauging diameter, parallelism and ovality 3-jet plugs for detecting lobed bores.


Open jet plugs should not be used on work which is porous or has a coarse surface finish.

Mechanical Contact Plugs

These should be used for gauging shallow bores or work which is porous or has a coarse surface finish.

Through Bore Plugs

These have the jet or contact positioned well back from the front of the plug.

Blind Bore Plugs

These have the jet or contact placed as near to the front of the plug as possible.

Extended Plugs

These are used for gauging deep bores. Extensions for plugs up to 14mm/O.55in diameter must be requested at the time of order. Large diameters may be fitted later.

Depth Collar

This can be fitted to the plug body permitting rapid gauging to a defined depth. Depth collars can be supplied for all types except miniature plugs.

90ยบ Adaptor

This is useful for gauging bores while the work piece is still on the machine.


Miniature plugs are made from tungsten carbide or tool steel and all other types from Tool Steel or hard chrome plated. For high volume applications our Technical Sales Department will be pleased to advise you.