What part of the camshaft contacts the bottom of the lifter?

The camshaft is a critical component in an engine's valvetrain, and its design plays a pivotal role in controlling the timing of the opening and closing of engine valves. The cam lobe is the specific part of the camshaft designed to push against the lifter, initiating the movement required to open the engine valves.

When the camshaft rotates, the cam lobe follows a defined profile, which translates into the lift and timing of the valves. As the cam lobe rotates, it makes direct contact with the bottom of the lifter, transferring motion to the lifter, which then can act upon the rocker arm and pushrod, ultimately controlling the valve's movement.

The base circle, while a part of the camshaft, represents the section of the cam that does not produce any lift. It’s essential for the cam’s operation but does not contact the lifter. The rocker arm and push rod are linked components that move as a result of the interaction between the cam lobe and the lifter, but they do not directly contact the camshaft themselves in the same manner. This makes the cam lobe the correct component responsible for that crucial interaction with the lifter.