That's the "sleeve" that's press-fit into the head that the valves slide in... After many miles they get worn/oval and the valves get sloppy and beat up the seats more... New guides keep the valves straight up and down...
My head guy presses out the old ones and installs new ones, then machines them to size to fit the valve stem...
I helped a local member rebuild a "rebuilt" 340 this year, the guy didn't have hardened seats for the exhaust valve, so we took them to my head guy, who found that the valve guides were worn also and needed replacing... He took care of it...
I have a great head guy in Michigan near Detroit. He found out how the head guy for the one local race team was doing his valves, then said, "Let me do a set for you..." He did a set of heads for a race team that usually ran the 1/4 mile in the 6's, and his heads got them into the 5's.... He also does valve jobs on small private airplanes... I moved to Chicago, and still bring all my head work to him... When you find a good machine/head guy - stick with him like glue...
