Rich, I have done the 8v stem seals in situ and it is not to bad a job. I made a adaptor out of a old compression tester and coupled it up to my compressor and let it have 100psi to hold the valves shut. Bit tricky getting the seals of the guides, long nose pliers worked for me. As for using it as a leak down tester, good idea, You can check for air coming out the exhaust, inlet to check the valve seating and the oil cap for rings etc.
Gaz, as for stem seals on a 16v in situ..., never tried that....yet

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Steve.