O-ring tools are components that help with the installation and functioning of an O-ring. An O-ring is a gasket or seal in the form of a ring with a circular cross section, typically made of rubber and used for swivelling joints. O-ring gauges: O-ring gauges help to determine the right size of O-ring to use. They have a series of silhouettes to the side of the gauge that identifies small section O-ring sizes. Larger sizes are determined by placing the O-ring around the two halves of the appropriate spindles at the top of the gauge. The slide is then extended and the size number is displayed in a window of the main body of the gauge.O-ring testers: An O-ring tester is a compact tool that performs a non-destructive field test of an O-ring by releasing a weight that falls to a set distance to indicate the material composition. O-ring testers are typically made from durable stainless steel which allows them to be carried safely in tool boxes.