A reciprocating test rig driven by a slider-crank mechanism has been developed for the purposes of generating piston ring friction coefficient data needed for engine modeling friction loss computations, and for performing basic tribological studies of piston engine moving components, including the piston skirt, in matters of quantity and detail. The ring friction coefficients were measured along a realistic stroke length with the ring in a compressed state and motion in the groove preserved. Ring and liner specimens were prepared from actual engines to ensure that the surface finish and material metallurgy are realistic. A data acquisition and analysis system was also developed. Results are presented in looping type Stribeck friction coefficient vs. Sommerfeld number relations directly applicable for engine modeling friction loss predictions. Such relations can also be used to examine the basic ring friction behavior quantitatively.