Dodge D250 1990 Service Manual page 356

Base Engine Components Alternative Repair Procedures
OODGE Turbo Diesel Page 9K-25
Install the wives in their respective locations and measure the
depth.
Valve Depth Limit
C = 0.99mm to 1.52mm [.039 to .060 inch]
Apply a light coat of valve lapping compound to each valve and
lap each valve to its mating seat.
Remove the valves and clean the lapping compound from the
valves and seats.