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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. 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. 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.