Set the timing mark to 0 TDC #1.

So; how do you know if you are TDC #1? ( rotating an engine is MUCH easier when there are no belts installed and all the plugs have been removed ! ) Rotate the crank clockwise and watch the rocker arms over the #1 cylinder. If the #1 rockers are moving when the timing mark lines up to the tab the engine is firing on # 6.  You will need to rotate the crank another full turn to set the engine to fire on the #1 cylinder. If you get lost or are unsure......  remove the number 1 spark plug and rotate the crank a couple more times.  A clear indication that the engine is firing @ #1 is that engine compression will be forced from the #1 cylinder spark plug hole, so  feel for air being forced out of the hole as you rotate the crank. ( And the #1 rockers will NOT be moving as the timing mark reaches the timing tab !)

Once you are sure that the engine is at #1 TDC the following valves/rockers can be adjusted:       

exhaust - #1, #3, #4, #8
intake - #1, #2, #5, #7

First you must find "zero" lash. This step takes all slack out of the lifter-rod-rocker assembly.  This is accomplished by grasping the rod and "jiggling" it so it clanks between the valve and the rocker arm.  Tighten down the retaining nut ( by hand if possible) until the push rod can no longer "clank" between the valve and the rocker.  THIS IS ZERO LASH!  (I do NOT suggest "spinning" the push rod because the pushrod can/will continue to spin after 0 lash is reached !) As soon as 0 lash is reached, turn the rocker arm nut down 1/2 to 3/4's (max 1 full turn) of a turn in 1/4 turn increments. A 1/4 turn increment is recommended to set the lifter pre-load.

Rotate the crank 360 degrees and set the other half of the valves
exhaust - #2, #5, #6, #7
intake - #3, #4, #6, #8

Note: If you are working with an older engine or used lifters it's possible that the lifters bleed may off pressure. (soak lifters for 1/2 hour in clean engine oil before installation.) You can improperly adjust your valves/rockers if you are not aware of this. You should have oil in the lifters to to get the most accurate setting. If the lifters have bled off their pressure than they will compress slightly after 0 lash is reached. (Once reaching 0 lash you can push the rod end of the rocker with the palm of your hand.... this could make finding 0 lash tricky !) Rockers can still be adjusted - just remember that ZERO LASH is achieved as soon as all slack in Lifter-Rod-Rocker is removed !

**  Beware of using a socket that makes contact with the rocker arm in ANY WAY. 

**  Most GM Factory Shop manuals I've read call for a 1 full turn adjustment past 0 lash.  Many aftermarket rocker arm companies call for 1/2 turn for final adjustment. A Rule of thumb I use is that if the all parts are new the adjustment is a a bit smaller and if the parts are used the adjustment is a bit larger. I have always had good luck using 3/4 of a turn, but I also re-use a lot of parts !!  follow the instructions for your exact parts to be sure !