Ducati MONSTER 821 Workshop Manual page 939

Heat the cylinder head gradually and evenly up to 150 °C and chill the new valve seats in dry ice.
Drive the seats perfectly square into the head housings using the appropriate valve guide seat installers
88713.2846 and 88713.2847.
Allow the cylinder head to cool down and grind the seats to the following dimensions: CA = Ø 41.6 ± 0.025
mm. CS = Ø 33.6 ± 0.025 mm. S = 1.2 mm. D = 0.2 to 0.4 mm.
Important
Do not use any lapping compound after final grinding.
It is advisable to lap the radius between the intake valve seats and the intake ducts (F = before; G =
after).
939 Ducati Manuals Resource