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If you measure the depth of the valve heads into the combustion chambers I expect it will account for the variation in chamber volume. Those chambers with thicker new valves or seats that are not as deeply cut into the heads will have the smallest volume.
As to how much variation is acceptable, that depends on how close you are running to "too much" compression as determined by the octane of your fuel, or in some case the racing rule book. At 10.5+:1 your have a lot of compression for pump gas. In this case equalizing the chamber volume and possibly increasing it may be desirable. Some racers will sink the valves a bit on the small chambers to do this, although sinking valves very much is generally undesirable.
Unless restricted by a rulebook some chamber modification my better serve to equalize and increase the volume. The ridge on either side of the spark plug and the chamber wall around the intake valve in order to "unshroud" it are suggested. Don't screw up the head gasket sealing surface. drop in an old valve to protect the valve seat while grinding. Try to have a minimum reduction of the area that provides "squish". Use eye protection when grinding.
Good luck
Charlie
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