Thanks, I didn't think about that! Haven't talked to the workshop yet, about measurement in general, of valves and piston. Shouldn't I be able to use my feeler gauges? The smallest size on my set is 0.05 mm ~ 0.02 inches. I was a bit reluctant with taking the valves apart. My Haynes' service manual is a bit scary with a lot of details on that part, but now I understand, that the worst part is repairing and reinstalling after a 'grounding' - and what is that? Cleaning the valves, is that grounding them, in a way where I should apply new lapping compound on the heads? As I understand it, I should take them out of the head, clean them carefully, including the combustion side, to be able to inspect and measure it. Hopefully, no problems are revealed, and then it's a simple 'reverse your steps' re-install.