This paper is designed to aid and guide the automotive engine designer in selecting a piston design capable of performing satisfactorily with particular requirements. Design parameters relative to clearances, strength, heat flow, and control of expansion are considered. The correlation of these factors to practical foundry practice and production machining requirements is vital to successful design. Guidelines are established for this correlation based on actual practice and experience. A review of new techniques designed to help the piston engineer pretest and outguess the engine is included. Finally, analysis of engine test results and procedures for corrective actions are suggested.