NvK, after you lap them, assemble the valves and springs and check the seal with a vacuum pump. A hand held Mighty Vac is perfect. I use a Mighty Vac hand pump and a port adapter I made up out of a rubber cup. If the valve will pull down vacuum and hold it for a minute or two, it will seal and hold compression. All good machine shops vacuum check the valves before they leave. It is the best way to know if it is going to seal.