Hey Gents! Still have to do valve stem seals on our 2004 120i too.
The only reason why the cylinder head has to be removed, is for the mech to be able to compress the valve springs with a huge G-clamp like tool, which fits right over the cylinder head when in the work bench. Remove springs, get to the small rubber seals, clean/lap valves, replace seals, put all back together again.
With the 'special tool', it enables the mech to compress those springs on the cylinder head where it sits on the engine, without having to remove the head, but the tool must be right in order to make it work. (that spring tention is not to play with!) There will be no cleaning/lapping valves then. Also not always necessary in each case.
The valves can be set & cleaned when the head is off while his at it. Makes everything seal 100% again for that true combustion = performance. :=):
That's a rough explanation of the process, either way, it's a lot of labour time to do it! That's why so expensive to have it done.
Hope that helps with the 'how it's done' stuff. :fencelook: