GM Firebird 1967-1981 Repair Guide

Valve Lash Adjustment


Except 250, 305 and 1978-79 350 Engines

The purpose of the hydraulic valve lifter is to maintain zero clearance in the valve train. It does this by expanding to take up additional clearance created as the lifter moves onto the base circle of its camshaft lobe. To perform properly, the lifter must be adjusted halfway between its fully-extended position and its collapsed position. If the lifter is adjusted too loosely, the valve will not open fully; if adjusted too tightly, any number of major mechanical failures may result.

When a rocker arm is loosened or removed, the lifter will expand to its fully-extended position. Upon reassembly, it is necessary to be sure that the lifter is on its camshaft lobe base circle before adjusting the lifter. This is the purpose of the preliminary valve adjustment. Perform the final valve adjustment after the engine is running.

  1. Rotate the crankshaft until the No. 1 piston is at TDC on the compression stroke and the distributor rotor points to the No. 1 spark plug wire cap tower. The timing mark should be aligned with zero (0ordm;) on the timing cover.
  3. Tighten the adjusting nut until the play just disappears. Adjust both valves. On Ram Air and Super Duty engines, adjust the valves to obtain a 0.008 in. clearance between the rocker arms and valve stems, then tighten the adjusting nut an additional 1 turn and tighten the locknut.
  5. On V8s, rotate the crankshaft 90ordm;, in the normal direction of rotation, to bring the next piston in the firing order to TDC on the compression stroke. Repeat Step 2 until all of the valves are done. On 6-cylinder engines, bring each cylinder to TDC on its compression stroke (every 120ordm; of crankshaft rotation) and repeat Step 2.


On Ram Air and Super Duty engines, refer to the valve "Lash Adjustment" procedure in Engine Performance And Tune-up for the final adjustment.

  1. Start the engine and retighten the rocker arm on any valve that is clattering. Tighten until the noise disappears.
  3. Allow the engine to run until the normal operating temperature is reached, then loosen each rocker arm adjusting nut until clattering begins. Retighten the nut until the noise disappears. On all V8s tighten the nut, very slowly, to 20 ft. lbs. On the 1970-72 6-cylinder engines, very slowly tighten the nut 1 / 2 -1 revolution further.

The purpose of tightening the adjusting nut slowly is to give the lifter time to adjust its height.

250, 305 and 1978-79 350 Engines See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: To adjust lash, loosen or tighten the rocker arm nut except 231 and OHC engines

  1. Remove the rocker arm covers and gaskets.
  3. Adjust the valves on 6-cylinder engines as follows:
    1. Mark the distributor housing with a piece of chalk at No. 1 and 6 plug wire positions. Remove the distributor cap with the plug wires attached.
    3. Crank the engine until the distributor rotor points to No. 1 cylinder and No. 1 piston is at TDC (both No. 1 cylinder valves closed). At this point, adjust the following valves:

      No. 1 Exhaust and Intake
      No. 2 Intake
      No. 3 Exhaust
      No. 4 Intake
      No. 5 Exhaust

    5. Back out the adjusting nut until lash is felt at the pushrod, then turn the adjusting nut in until all lash is removed. This can be determined by checking pushrod end-play while turning the adjusting nut. When all play has been removed, turn the adjusting nut in 1 full turn.
    7. Crank the engine until the distributor rotor points to the No. 6 cylinder chalk mark and No. 6 piston is at TDC (both No. 6 cylinder valves closed). The following valves can be adjusted:

      No. 2 Exhaust
      No. 3 Intake
      No. 4 Exhaust
      No. 5 Intake
      No. 6 Intake and Exhaust


  5. Adjust the valves on V8 engines as follows:
    1. Crank the engine until the mark on the damper aligns with the TDC or 0ordm; mark on the timing tab and the engine is in No. 1 firing position. This can be determined by placing the fingers on the No. 1 cylinder valves as the marks align. If the valves do not move, it is in No. 1 firing position. If the valves move, it is in No. 6 firing position and the crankshaft should be rotated one more revolution to the No. 1 firing position.
    3. The adjustment is made in the same manner as 6-cylinder engines.
    5. With the engine in No. 1 firing position, the following valves can be adjusted:

      Exhaust Nos. 1, 3, 4 and 8 cylinders
      Intake Nos. 1, 2, 5 and 7 cylinders

    7. Crank the engine 1 full revolution until the marks are again in alignment. This is the No. 6 firing position. The following valves can now be adjusted:

      Exhaust Nos. 2, 5, 6 and 7 cylinders
      Intake Nos. 3, 4, 6 and 8 cylinders


  7. Reinstall the rocker arm covers using new gaskets or sealer.
  9. Install the distributor cap and wire assembly.
  11. Adjust the carburetor idle speed.