[Walt Fricke]

During the post machining stage of a 911 valve job, the valve closure integrity is typically done with air pressure, either in the port with some kind of plate and fitting for seal, or with a plate over the combustion chamber and the air fitting in it. The seal doesn't have to be fancy - a rubber gasket is fine. Then fill the other side with water, or some soapy water, and watch for bubbles. You can pressurize it by blowing through a hose, and hold things on with your hands. If the valve to seat seal isn't competent, this will show it. It won't show if everything is as you want it - maybe the sealing surface is too narrow or something. But no amount of pressure will show that.

Some just pour water or some equally thin fluid in and see if gravity pushes any past the recently done valve. You can do this stuff without the valve springs, too, just holding the valve firmly in place.

Maybe Jeff remembers what he sees for leakage numbers on the engine stand. I think I did this once long ago, but forget what the numbers were.