See Figure 1
With the exception of certain high-performance V8 engines (289 High-Performance, Boss 302, Boss 429, Boss 351, 429 SCJ, and 351 HO of 1972) all engines used have been equipped with hydraulic valve lifters operate with zero clearance in the valve train and, because of this, the rocker arms are nonadjustable. The only means by which valve system clearances can be altered is by installing 0.060 inch over or undersize pushrods; but, because of the hydraulic lifter's natural ability to compensate for slack in the valve train, all components of the valve system should be checked for wear if there is excessive play in the system.
When a valve in the engine is in the closed position, the valve lifter is resting on the base circle of the camshaft lobe and the pushrod is in its lowest position. To remove this additional clearance from the valve train, the valve lifter expands to maintain zero clearance in the valve system. When a rocker arm is loosened or removed from the engine, the proper valve setting is obtained by tightening the rocker arm to a specified limit. But with the lifter fully expanded, if the camshaft lobe is on a high point it will require excessive torque to compress the lifter and obtain the proper setting. Because of this, when any component of the valve system has been removed, a preliminary valve adjustment procedure must be followed to ensure that when the rocker arm is reinstalled on the engine and tightened, the camshaft lobe for that cylinder is in the low position.
See Figure 2
This procedure for the 289 and early 302 V8 engines is designed for engines in which the rocker arm mounting studs do not incorporate a positive stop shoulder on the mounting stud. These engines were originally equipped with this kind of stud. However, due to production differences, it is possible some 289 or early 302 engines may be encountered that are equipped with positive stop rocker arm mounting studs. Before following this procedure, verify that the rocker arm mounting studs do not incorporate a positive stop shoulder. On studs without a positive stop, the shank portion of the stud that is exposed just above the cylinder head is the same diameter as the threaded portion, at the top of the stud is of greater diameter than the threaded portion, this identifies it as a positive stop rocker arm stud and the procedure for the 351 engine should be followed.