Valve adjustment should be performed anytime the heads are removed. It's also good practice to adjust valves during service that require valve cover remove.
First start with, locating the timing mark on the front engine cover and the harmonic balance.
To adjust the valves, start by loosening all the jam nuts with a 14mm wrench, you will also need a flat screwdriver to make the adjustments.
Adjust the valve using a .012" feeler gauge Use your .012” feeler gauge between the rocker arm and the valve bridge as pictured. The correct setting is had when there is slight resistance when moving the feeler gauge. It should not be so tight that the gauge can not be removed. Once the proper setting is made, tighten the jam nut while holding the adjustment screw. Verify that your setting has stayed after tightening the jam nut. After going through all the valves, rotate the engine over about 10 times by hand in a clockwise direction. Now, bring cylinder number 1 back to TDC and recheck your adjustments. You may find that some have loosened up a bit, go ahead and readjust them to spec.
Toxic Diesel Performance
General Motors Company