A brief survey is given of previous investigations into piston-boss and wrist-pin design, and it is concluded that although a theoretical stress analysis of the pin is possible, the use of such analysis is limited by assumptions which have to be made regarding the effect of case-hardening the pin surfaces. A description is then given of fatigue tests carried out by the author’s company on wrist pins to determine the effects of various materials and alternative heat-treatments. The result of the investigation enables lower-grade materials and cheaper methods of production to be used, while at the same time enhancing the overall life of the wrist pin. Investigations employing photoelastic and brittle-lacquer stress analysis of piston-boss and boss-support webs are described, leading to recommendations for the detail design of the boss, boss-support, and wrist-pin assembly. In view of the necessity for engine outputs to be increased in the near future, it is considered that there is a need for the wrist-pin assembly to be reconsidered in order to increase the load-carrying capacity. It is concluded that each piston design must be subjected to both rig and engine tests and that continued investigation will eventually lead to improved designs capable of withstanding higher gas loads without a proportionate increase in weight.

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