Abstract: A fuel cap tester kit containing a plurality of fuel cap adapters, wherein each adapter has a mouth opening, outer rim, cap receiving chamber, air supply passageway and a fuel cap connection opening and with each fuel cap opening being configured to engage and retain a different and specifically configured fuel cap, and wherein each adapter is also provided with a threaded base connection that cooperates with a complimentary threaded connection on a test base that is fluidly connected to a pressure source and tester apparatus and wherein the base connectors of each adapter are configured alike so that each adapter can be connected to the single threaded connector in the test base to permit testing of different size fuel caps dependent on the adapter threaded into the test base, where the tester kit is used to decide upon quality of fuel caps under test by determining a leakage rate of pressurized air from the cap-receiving chamber through the mouth opening.