Motoring Correspondent Andrew English (who races a DB5) writes: “Most valve seats are made of a colbalt chromium steel alloy known as Stellite, which is tough rather than brittle and has excellent wear
properties. They are press fitted, screwed or stitched into place. The Aston has such valve seats in its aluminium cylinder head and the recession is negligible, which is just as well as the valves have to be machined to set the clearances.”