Here's why:

* End play refers to the amount of axial movement a shaft can have within its bearings.

* A dial indicator is a precision measuring tool that measures very small displacements. It consists of a dial with a needle that moves in response to contact with a surface.

To measure end play, the dial indicator is mounted to a stationary point and its probe is brought into contact with the end of the shaft. As the shaft is moved axially, the dial indicator measures the change in distance, which is then used to determine the amount of end play.

Other instruments that might be used in certain situations include:

* Micrometer: Can be used for more accurate measurement of end play if the shaft is stationary.

* Laser displacement sensor: Can be used for contactless measurement of end play.

Let me know if you'd like more information about measuring end play!