As a poppet valve receded under normal use, the valve operating clearance ____________.

In the context of engine mechanics and specifically focusing on poppet valves, as a poppet valve recedes or moves away from its seat during operation, the valve operating clearance tightens up. This occurs due to the thermal expansion of engine components and wear over time.

When an engine runs, it experiences heat, which causes the materials, such as the valve stem and the valve seat, to expand. However, as the valve operates, there might also be wear on the valve seats and the tappets, leading to changes in the valve clearances. Importantly, as the clearance decreases, the engine can experience an increase in performance, but may also lead to issues such as valvetrain noise or increased wear if the clearance becomes too tight.

In summary, with the normal operation and conditions that affect a poppet valve, it's evident that as it recedes, the operating clearance becomes tighter, which is key for ensuring the valve's efficiency and longevity in operation.