The Home Machinist!

How do you seat your valves?
Mine is 0.25" dia. with a seat about 0.020"

What Mesh grit do you use?
I have carbide grit 500, 400 and 180 on hands
Do you use oil or dry?

Install your valves and don't lapp them. If you feel you have too, use 2000 grit or higher. You will want a mirror finish. Most people actually make the seal worse by lapping with a harsh grit. Personally I cut my seats and if I get a nice finish I don't lapp. A good finish on the valves and seats and you shouldn't need to lapp.

Trust me, I have made my fair share of 1/4 inch valves and cages.

As Steve has mentioned he has done many many valves and seats and in the ideal world if the machining is accurate the valve should seal right from the start but usually that's not the case. Most of my engines have the seats and guides cut into the head (no valve cages) so I use a different approach in trying to get them as accurate as I can. Usually I have pretty good luck but there's always a couple that won't hold a vacuum so I end up lapping them. When I do I use the finest grit lapping compound that I have. It generally takes no more than a handful of rotations to clean up the seat, if it takes more then something is wrong. Another thing that will affect a good seal is the clearance between the valve stem and guide. This should be as close as possible but still allow smooth movement (.00025-.0005)
gbritnell