A pair of outside micrometers is the most accurate.
Someone told me a micrometer?
A piston is a plunger, or a plug that moves inside a cube, a cylinder. The piston diameter is the diameter of this plunger/plug. It's a tiny bit smaller than the diameter of the cylinder the piston moves in.
Diameter tools
diameter
depends on your definition of "smallest" depends on your definition of "smallest"
Use a tape measure !
First of all it's "What unit of measurement would you use to measure the diameter of a drink can?" Secondly you would probably use either centimeters or inches, depending on where you are in the world.
4"
use an ometer
centimeters or millimeters
The numbers like .75 .50 indicate the oversize of the piston ring that will be used for repair. For instance if the engine has a diameter of 70 mm and you need to open the cylinder bore for repair in .50 mm, you need to use one piston ring with diameter 70.50 mm.
A diameter is measured by using a ruler. However, you also could use a digital caliper to measure the diameter of a circle.