The process of reducing the high spots by rotating valves against the seats - or any similar rubbing action of 2 surfaces is technically lapping. Whether the engine does it or a man is not part of the definition as I understand... but I may be wrong? (I am more often wrong than I prefer to admit!). But I was taught that the final "lapping" is done by the engine when running-in.... - But that was by a 50-years machinist, who was teaching me as a teenager... Sorry if that isn't correct, it is only what I know...
Incidentally, the same applies to crankshafts: Again, I was taught to machine cranks on a dedicated crank grinder - on anything from a 900cc to "large 6-cylinder" diesel cranks. The grinder goes to a certain surface finish (the carborundum stone grade) then the surface is lapped by the finest grade of emery (carborundum on cloth). I was taught the final surface lapping was done "by the detritus in the oil" during running-in as the oil passed through the bearing to crank gap. and the result was the dirt in the oil seen at first oil-change and washout after running-in. Hence, Molybdenum can effectively "damage" an engine as it prevents the "lapping" from happening in the running-in phase.
But Then an Engineer from Glacier-Vandervell Bearings taught me that Aluminium-tin bearings were appropriate on cast-iron cranks, as - from the SEM study - the grinding (Manufacturer's factory finish) left "little hooks" of bent microscopic bits of iron... The Al-Sn alloy contained Silicon - which "lapped" the surface of the cast-iron and removed these "hooks", as well as any other high-spots or hard "lumps" in the surface. If the cranks were ground "the wrong way", then the hooks acted like tools and cut microscopic grooves in the bearing material... But the grinding machines or engines would have to run backwards to do that. Apparently when they first made such Al-Sn bearing materials there were rare cranks made the normal handed way, but which ran backwards ... (e.g. in multi-crank engines?) - which is how they came to understand what was happening (hence the reason to look with the SEM)... Apparently (if my memory is correct?) the crystalline structure of forged steel cranks is different from cast-iron so these "little hooks" do not appear... therefore these journals are more suited to plated Phosphor-bronze bearings. (The running-in of cast iron on the plating of Phosphor-bronze bearings would destroy the plating - so the bearing would be prone to corrosion damage later in life). So crank materials have preferred bearing materials. Anyway, I remember (if imperfectly?) the reference to "lapping" of cranks in the engine...
Sorry if I am wrong, but none of us are perfect learners, nor teachers. What do you know? - I'd like to learn.
