Hydraulic Lifter Preload
To understand hydraulic lifter preload, a comparison could be made between a solid lifter and a hydraulic lifter. Manufacturer’s specifications are required to set the gap on either of these lifters.
A feeler gauge is used to measure the gap between the valve tip and the rocker of a solid lifter. This gap is necessary to accommodate heat expansion in the valve train.
Adjusting hydraulic lifters is different. With a hydraulic lifter, the gap is set to zero lash, and then the preload is set. This initial zero lash adjustment is why these lifters have a quieter operation.
This is accomplished with the use of a spring loaded plunger mechanism inside the lifter body. When preload is being set, the plunger is being pushed into the body. Excessive valve train noise will result from insufficient preload. Low manifold vacuum or a rough idle will result from excessive preload.
A sticking plunger will likely result in a burned exhaust valve or an intake valve that does not completely close. 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. When unsure about something, always use manufacturer’s specifications. 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.