Thread gauges are mechanical devices used to measure the thread pitch and profile of a screw. It helps to determine the number and size of the indents on a screw.

1. Thread Gauges: Units, Types, Uses in Industry
Thread gauges are mechanical devices used to measure the thread pitch and profile of a
screw. It helps to determine the number and size of the indents on a screw. The indents,
also known as the threads of a screw, have to match the indents inside the bolt to be
compatible.
This device is made of hardened stainless steel which is also water-resistant in order to
combat moist weather conditions. There is an assortment of metallic leaves, usually 28
leaves on each side, both in imperial and metric units for the operator’s convenience. The
leaves are indented with different thread sizes to measure the appropriate screw thread
dimension.
The leaves can be tightened with a screw attached on either side of the metal bar. Once a
screw is brought in for measurement, the leaves with similar thread numbers are run along
the screw threads. Whichever leaf fits perfectly into the screw thread determines the
measurement of the screw thread.
Units: imperial or metric?
Values obtained from thread gauges can be read in either imperial or metric unit. The
operators generally measure the screw thread in terms of Threads per Inch or TPI, which is
the imperial value. This value implies the number of indents the respective leaves have per
inch. Thus, if the number of teeth on a corresponding leaf is 12 per inch, the measurement
is denoted by 12 G.
The metric unit system uses a whole number to denote the value which is the imperial
equivalent of the screw pitch.
Thread plug gauges for bolts
A regular thread gauge is used to measure the external pitch of a screw, while a thread plug
gauge returns the internal pitch of a bolt. The threads of a bolt are placed on the inner
surface, where the thread plug is inserted for measurement. A screw is compatible with a
bolt only when the thread gauge value and the thread plug gauge value coincide. This device
is usually double ended, while single ended thread plug gauges are also in use.
Applications in industry
Thread gauges and thread plug gauges are used in various industries such as:
 Construction
 Aviation and automotive

2.  Industrial machinery
 Pumps and motors
Since screws and bolts are indispensable in any industry, the dependence on thread gauges
and thread plug gauges is considerable. This device measures the screw pitch accurately so
that there is a proper classification of screws and bolts according to dimensions. The value
returned by thread gauges may also imply whether a thread seal is to be used to ensure that
system fluids do not leak through the flanks.