These adapter fittings are available in a variety of configurations. They push-on and secure to most modern USA, European, Japanese, etc. fuel injection systems and are used to convert an OEM SAE Quick Connect connection.

MEASUREMENT
To determine the required size, simply measure the outside diameter of the male fitting as shown below.
0.3125 = 5/16″
0.3750 = 3/8″
0.5000 = 1/2″
0.6250 = 5/8″