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Roller tappets are commonly used in automotive valve trains to transform the rotational motion of the cam shaft into axial motion of the valves. Proper design of the cam and roller tappets requires an understanding of the factors that influence the integrity of the contact surfaces. Ideally the roller tappet to cam lobe contact should exhibit pure rolling (equal surface velocities). However, relative sliding (unequal surface velocities) occurs under most conditions. Relative sliding of contacting surfaces operating under marginal lubrication can contribute to deterioration of the surfaces. The relative sliding of tappet roller to cam has been experimentally determined by means of a simulator rig. Sophisticated instrumentation has been used to measure the velocities and synchronize them with the lift event. The experimental results showed high sliding during contact with the cam base circle (up to 20%).