Hydraulic Lifter Preload
Hydraulic lifter's require a slight preload. The manufacturer's specifications are required to set the gap between a solid lifter and the cam lobe. A feeler gauge is used to measure the gap between the valve tip and the rocker arm of the solid lifter. This gap is necessary to accommodate heat expansion in the valve train.
Adjusting hydraulic lifters is different. With a hydraulic lifter, first set the gap to zero lash, and then set the preload. This initial zero lash adjustment prevents valve tap and is why these lifters have a quieter operation. With zero backlash the pushrod should easily rotate in your fingers but have no up and down movement.
There is a spring-loaded plunger mechanism inside the lifter body. The preload is pushing the plunger slightly into the body. If the is preload out of specifications, valve train noise, and a rough idle result. Insufficient preload results in valve train noise, excessive preload results in a low manifold vacuum, and a rough idle.
A sticking plunger will result in a burned exhaust valve or an intake valve that does not close completely. A sticking open intake valve will cause compression to backfire into the intake manifold.
Adjusting Hydraulic Lifters
1. To set the lifter preload, turn the engine in its normal rotation until the exhaust lifter just starts to travel upwards.
2. Adjust the intake valve to zero lash and then turn the rocker arm adjustment nut 1/2 to 1 turn. This additional turning of the adjustment nut sets the lifter preload.
3. Rotate the engine until the intake lifter travels all the way up and then almost all the way down.
4. Now set the exhaust valve to zero lash and then turn the rocker arm adjustment nut 1/2 to 1 turn. When unsure about something, always use the manufacturer's specifications.