According to the factory manual, exhaust valves should be lapped in every 1,000 hours and intake valves should be done every 3,000 hours.
Intake and exhaust valves sit in removable cages that are held in the head by two studs, and are identical except that the intake valves do not have cooling water supplied to the valve cage's water passage through copper tubing as on the exhaust valves. This means that in order to remove an exhaust valve the engine must be partially drained of water down to the level of the valves.
In order to make the task of lapping the valves easier, we made a stand for holding the valves and for compressing the springs for removal.
Here is a picture of a valve before and after lapping: The spring is used to hold the valve off the seat while lapping it in.
Larry, the Oiler, starts on one of the 48 valves awaiting him:
This picture shows the components of a disassembled valve cage:
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