This study investigated the effects of piston Crevice geometry on the steady-state engine-out hydrocarbons (HC) from a Saturn DOHC four-cylinder production engine. A 50% reduction in top-land height produced about 20-25% reduction in HC emissions, at part loads. The effect of top-land radial clearance on HC emissions was found to depend on the value of top-land height, which suggests a complex relation between flame propagation in the piston crevice and crevice geometry. For idle, increasing top-land clearance resulted in an increase in HC emissions. This trend is opposite to the trend at part load. A simple model was developed which predicts surprisingly well the contribution of piston crevices to HC emissions. It was estimated that for the test engine, piston crevices contribute about 50% of the engine-out hydrocarbons. Finally exhaust gas recirculation appears to decrease the sensitivity of HC emissions to crevice dimensions.