this was explained many moons ago with, from what i can remember, the same sort of pics

i was always under the impression that once it was skimmed into the leveo of the valve seat it was over the aceptable limit to use but no one has, as far as i have seen or can remember, explained why this was the case

if there is a proper reason rather than just being "becaue someone said so" it should stop all the issues surrounding this