The valve clearances on this motor do not normally require adjustment; as they are controlled automatically by hydraulic valve lifters. However; the valve train may become noisy as a result of dirt getting into the lifters, or the rocker arms may wear down at the contact surfaces and loosen up. I have seen engines where the pushrod wore completely through the rocker arm. In cases of severe wear, adjustment will not stop the noise. The worn parts must be replaced.
(non-roller lifter) Chevy motors will sometimes tend to wear down camshaft lobes. When a cam lobe goes flat from wear; that cylinder will lose power and the engine will miss at idle. The camshaft AND LIFTERS must both be replaced to produce a lasting result. GM did a lot of research on this problem; and found that this engine design is unusually sensitive to lubricating oil types. GM then published bulletins stating that 10W-40 oil is the worst type of oil to use in a small block Chevy, and that GM will not honor warranties on engine failures when 10W-40 oil is used. The oil type they recommended for non-roller motors is 10W-30.
When a motor is reassembled after a repair or rebuild; it is necessary to adjust each rocker arm height so that the plunger in the hydraulic lifter is positioned midway in its travel. The adjustment is made by turning the nut on the stud in the center of the rocker arm.
On a freshly assembled motor; the procedure for initially setting the valve clearances is to mark the FRONT FACE of the crankshaft pulley with a yellow chalk line pointing to the timing mark (so the timing mark location can be visible when the pulley is in any position). Then turn the crankshaft until the timing marks line up at TDC, and the distributor rotor points to the plug wire terminal for # 1 cylinder. At that position; loosen the rocker arm nuts on both valves for # 1 cylinder, just enough so you can first begin to feel play between the tip of each rocker arm and the valve stem it touches. Then turn the nut on that rocker arm clockwise exactly 1 1/4 turns. Do this for both valves in that cylinder. Then turn the crankshaft pulley 1/4 turn clockwise, and adjust the valves in # 8 cylinder by the same method. Then turn the crankshaft pulley 1/4 turn further clockwise, and adjust the valves in the # 4 cylinder. Then turn the crank pulley 1/4 turn further and do # 3 cylinder. Keep turning the crank pulley clockwise 1/4 turn each time, and adjusting the valves in one cylinder each time; until you have adjusted the valves in all the cylinders; following the adjusting order (which is the same as the firing order) of 1-8-4-3-6-5-7-2.
After the valves have been statically adjusted by this method; start the motor and let it get fully warmed up. It may be necessary to drive a newly assembled motor for 10 or 20 miles before all the air works out of the lifters and the noise all goes away. If it is then as smooth and quiet as you'd like; no additional adjustment is necesssary.
If you want to fine tune a valve adjustment on either a new motor or a used motor; you'll need to buy a set of rocker arm oil stop clips. These clips are designed to clip on to each rocker arm; at the end where the pushrod attaches. Their purpose is to stop the oil from spraying out of the pushrods and making a mess of the engine compartment while you run it with the valve covers off.
Start the motor and drive it until it is fully warmed up. Then remove the valve cover on one cylinder bank, and make sure the spark plug wires are all connected properly after the valve cover is removed. Attach the oil stop clips to the rocker arms; and start the motor. While the motor is idling; start at one end of the cylinder head and SLOWLY loosen the adjusting nut on the first rocker arm until you start hearing clatter from that valve. Then SLOWLY turn the adjusting nut clockwise 1 1/2 turns. It is important to turn the nuts slowly to allow time for the oil pressure in the hydraulic lifters to stabilize. Adjust the next rocker arm the same way, and continue down the row until all the valves have been adjusted. Then reinstall that valve cover and do the other bank of that motor.
Some motors will run smoother with the lifters adjusted to less than 1 1/2 turns down. The books say it can be anywhere between 3/4 turn and 1 1/2 turns. If the adjustment is set too far down (tight); it will make the motor run roughly. If it is set too far up (loose); the valves will make noise. I try to find the point where it runs the smoothest and the idle speed is fastest; without valve noise. But I recommend setting all the valves in the motor to the same distance (even though one or two may seem to run better if they are set differently) The minor differences you may notice often smooth out when the car is driven further.