This paper is concerned with the jump phenomenon, i.e. the separation of the members of the cam mechanism, which occurs at high speeds. For this purpose, an automobile engine's cam mechanism is taken into consideration and the amount of preset which must be given to the valve spring to prevent the separation is investigated. Two different models are used in formulating the problem. In the first, the pushrod is modeled as a continuous element, whereas in the second, it is considered as rigid. The valve spring is assumed to be a continuous element in both models. Results obtained from these models are compared and the accuracy of the models is investigated. All of the contact conditions are expressed analytically by equations in terms of nondimensional parameters. Comparing these equations, the most critical contact point is determined.