<div class="htmlview paragraph">AS an engine under development is not a tool which can be used for the testing and rejection of questionable spark-plugs, some test is necessary to parallel the engine development so that suitable spark-plugs are available for it when it enters service. This paper describes a preliminary single-cylinder engine test method for spark-plugs which evaluates the quality not only most essential but the most difficult to secure, namely, resistance to preignition.</div> <div class="htmlview paragraph">Tests were made on two types of engine-a liquid-cooled, single-cylinder test engine and an air-cooled, single-cylinder test engine. Operating conditions were selected with regard for satisfactory performance plus the ability to heat the spark-plug.</div> <div class="htmlview paragraph">A large number of plugs were tested, some of identical design, and many of them were re-run several times as a check on the reproducibility of the test method.</div> <div class="htmlview paragraph">In addition to the preignition test, a full-scale engine type test, a ground fouling test, a flight fouling test, and a starting-ability test are recommended, with bench tests at intervals and measurement of electrode erosion and wear as a check on the probable useful life and ability of the spark-plug to measure up to the requirements of flight service.</div>