This roller tappet converts the rotational motion of the camshaft lobe into linear motion to actuate the engine valve train. It sits between the cam lobe and the pushrod or rocker arm, and its roller reduces sliding friction compared to a flat tappet. A worn roller develops a flat spot, which causes valve timing drift and a rhythmic tapping noise from the valve cover. The original part is machined to the same roller diameter and shaft clearance as the production assembly, so no shimming or adjustment is needed during replacement. On installation, verify the roller rotates freely and lubricate the contact surfaces before torquing the retainer to specification.