I'm thinking TMM was suggesting that rather than use a pilot in the valve guide to machine the seats, they were somehow piloting off the OD of the guide.
That's the was I understood his question but I could be wrong about that.
I have seen several valve jobs with .050 TIR. They were all BBC's with 2.400 valves and .311 stems.
We don't know how sophisticated their machining centers are. If they are all cnc stuff they could machine the pockets for the seats and the bores for the guides in the same machine with a change of tooling. Just thinking out loud.
