How to lap a valve
The subject head is a Chevy LT1
The procedure will be the same for any cylinder head.
First thing to do is remove the Valve locks, retainers and springs.
Second is to clean the valves of carbon, oil and deposits.
I prefer the valve face to be ground to remove all pitting to insure a good valve to seat seal.
Most machine shops will grind the valve for a dollar or two.
The tools needed will be a lapping tool and valve lapping compound.
There are different brands, grits and bases. Even colors.
I use permatex water base fine grit. Grey in color.
Place a fair amount on your finger tip.
Then bead it around the valve face.
Next is slide the valve back down the guide and seat the valve.
Once the valve has made contact with the seat, stick the lapping tool onto the valve and spin the tool back and forth with both hands. You will feel and hear the valve grinding compound “crunching”. At random intervals lift the valve up so more compound will work down on to the seat. Appox.10-15 seconds.
Remove the valve and wipe clean and inspect the valve face, It should have a gray, or whatever color your compound is, line all the way around the face. IF not the valve is bent and should be replaced.
Next wipe the seat in the head clean, and look to see the same line around the seat, If you see areas on the seat that are not the color of the lapping compound, Load the valve again and lap some more till you have a perfect color line all the way around.
If you lap several times and the seat line does not come in, the head is severely warped and the seats are distorted, The head should go to a machine shop to be fully checked and have a valve job preformed If you have a good solid line around the seat, then clean all traces of the lapping compound from the valve and head and reassemble the valvetrain..