What is the main indicator that cleaning is required after lapping valves?

The main indicator that cleaning is required after lapping valves is the presence of grinding compound residue. During the lapping process, a fine abrasive compound is used to ensure that the valves properly seat against the cylinder head. After this process, it is essential to eliminate any leftover grinding compound because it can cause damage to the engine components, lead to improper sealing, and adversely affect engine performance.

If the residue is not removed, it can remain trapped between the valve and the seat, leading to potential leaks and other problems in engine operation. Proper cleaning ensures that the surfaces are smooth and free from contaminants, allowing for optimal sealing when the engine is reassembled. This highlights the importance of thorough cleaning as a crucial step to maintain engine integrity and performance.