How do I determine if I need to have the guides done on my 906 heads? I assume there is a measurement to check the play? The play in the valves seems pretty equal in all directions, with the exhaust valves seeming a bit loose compared to the intakes. When I cover the guide at the top and pull the valve out the intakes generate a very loud pop when they come out of the guide, the exhausts aren't as loud but still make a pop. I would think the exhausts might have a looser tolerance.

Also, the seats and original valves look to be in very nice shape. I want to replace them with one piece stainless valves though. What is the opinion on new valves on an old seat? Obviously a valve job would be better but is it necessary? I have hand lapped new valves into old seats before on a late model turbo application(~20 psi) with 115k miles and the car ran flawlessly. I would think if it could hold up on a boosted application it shouldn't be a problem naturally aspirated.