What is the process called where the valve face is rubbed against the valve seat using an abrasive compound?

The process where the valve face is rubbed against the valve seat using an abrasive compound is known as lapping. Lapping involves using a gritty paste that helps to create a smooth and precise seal between the valve and the seat. This technique is essential for ensuring that the valve closes tightly, preventing leaks that could affect engine performance.

During lapping, the valve is typically rotated against the seat with the abrasive compound, which gradually removes material from both surfaces. This results in a better mating surface, which is crucial for optimal engine efficiency and longevity.

The other processes mentioned, such as grinding and polishing, serve different purposes and are not specifically aimed at achieving the same fit and sealing quality that lapping does. Grinding is generally a more aggressive process used for reshaping or removing more significant amounts of material, while polishing usually refers to smoothening the surface after the primary shaping processes.