GRINDING OR LAPPING OF VALVES IS A NORMAL ROUTINE CARRIED OUT ON CYLINER HEAD FOR PROPER SEATING OF THE VALVE ONTO THE SEAT SO THAT AIR AND EXHAUST DO NOT LEAK OUT OF THE HEAD AND PROPER COMBUSTION, SCAVENGING AND EXHAUST WILL BE CARRIED OUT.
WHEN VALVE AND SEAT IS LAPPED/ GRIND THEN SOME MATERIAL IS REMOVED FROM THE VALVE OR SEAT DEPENDING ON WHICH ONE IS MADE UP OF LIGHTER MATERIAL. DUE TO THIS, SEATING FACES BECOME LOWER THAN THE SURROUNDING MATERIAL AND A STEP IS FORMED CIRCUMSTANTIALLY.
THE ACTUAL DISTANCE THAT THE VALVE OPENS IS SAME BUT THE EFFECTIVE OPENING AREA ACROSS THE VALVE AND THE SEAT FACE IS REDUCED BY THE AMOUNT OF THE SHROUD WHICH LEAD TO DISRUPTION OF GAS AND AIR FLOW ACROSS THE VALVE AND THE FORMATION OF EDDIES BY THE SHROUD CAUSE LOSS OF STREAMLINED FLOW.
