Valve lash adjustment. Remember #1 & #4 piston go up & down together only on different strokes. Same for #2 & #3. With the valve cover off, turn it over with a hand crank until you observe the #4 rockers. The exhaust (last one)coming up and the intake starting down (top of the exhaust stroke). At this point, observe the position of your hand crank and check to see if the TDC mark is in the timing hole. When you do all of this on #4 and everything is in place, both rockers for #1 can be set as #1 is on the top of the compression stroke (TDC). Then follow the firing order 1243. When #1 is set, back up the hand crank to catch the notch and then turn the hand crank 1/2 turn stopping in the same place and the rockers on #2 are ready for adjustment. Then 1/2 turn for #4 etc.
This may sound complicated but it only takes 2 revolutions of the crank to complete the adjustments. Also by always backing up the crank a half turn to catch the notch and then stopping the crank in the same position each time, you are close enough to TDC to make the adjustments.
Years ago our bolck man told me to set valve lash at .020 for heavy duty use. Both intake and exhaust. Duey