To surely prevent the rupture of a cam lobe due to excessive stress of an expanded pipe by providing a uniforming means for nearly uniform radial fastening force in the axial direction of the hollow shaft and the cam lobe in the case where the hollow shaft and the cam lobe are unified under mechanical fastening force.

In the case where a cam lobe 20 is unitedly fixed to a hollow shaft 30, the bore d1 of a central part CP of the cam lobe 20 and the bore d2 of both its end parts E1, E2 are machined so as to be d1<d2. When expanded pipe assembly is carried out using such cam lobe 20, the fastening force of the cam lobe 20 to the hollow shaft 30 becomes larger at the central part CP, and nearly uniformed in the axial direction of the hollow shaft 30. In addition, the hollow shaft 30 is machined so that the outer diameter d3 of the central part CP and the outer diameter d4 of both its end parts E1, E2 may be d3>d4. In the case where expanded pipe assembly is similarly carried out, the fastening force of the cam lobe 20 to the hollow shaft 30 becomes larger at the central part CP, and nearly uniformed in the axial direction of the hollow shaft 30.