A tired-ish engine with over 100,000 miles can be rejuvenated to rise again for another 100k run.
There was nothing physically wrong with this head, but a few valves would not seal adequately due to the carbon buildup that is inevitable with direct-injection gas engines. That carbon becomes so hard that it can pit or break steel and aluminum. It also impinges on airflow, effectively choking the combustion chamber. After successive attempts to clean out the carbon, it was still not good enough.
With machining and extensive cleaning, the head is then vacuum-tested to confirm its integrity. All related parts are refreshed as it goes back into the car, and it’s as good as new.