cylinder head

Lapping valve I don't see this in the course.

My thoughts - depending on the application, this is often counter-productive.

Thre are three main reasons for this - some cases there is a slight (1 degree) difference between the seat and the valve, with the seal being formed where they meet, the valves - especially the exhaust which is usually at least red hot - change size and shape when running and this moves them from the 'lapped' match, depending on the valve and/or seat material there may be slight deformation in service which affects the seat geometry.

IMO, it's more important to ensure the valve to guide clearances are minimised, and the valve is concentric to the seat - it's a good idea to always check the seal with some coloured solvent, though - especially if you don't have the tools to check concentricity.

Lapping valves is not really a technique any more, hence we don't cover it. The issue is that when you lap a valve you actually end up making the valve and valve seat convex/concave and while they will seal nicely at room temperature, once they expand at operating temperature the valve seal can be pretty terrible.

The only caveat I'll add is that a very light lapping with a fine grinding paste can be useful to visualise the contact area and width of the seat but it isn't a technique we would recommend for actually seating the valve or for recovering a poor valve seal.