Pump It Up. Preloading Is Important!

Hydraulic LiftersMarch - April 1999 Motor State Performance Report Article By Jim Kaekel, Jr.

Hydraulics lifters are used in the majority of modern automobiles because they are reliable and maintenance-free, with a long service life expectancy. They rarely need adjusting, and will provide quiet, trouble-free operation for years.

The function of any lifter, hydraulic or mechanical (solid) is to ride the eccentric slope of the camshaft's lobes and effect the opening and closing of the intake and exhaust valves. Hydraulic lifters have the ability to automatically adjust themselves and maintain zero clearance in the valve train by supporting the pushrod plunger on a small chamber of pressurized oil, regulated by a precisely controlled oil bleed rate. Valve train noise is prevented because the lifter is preloaded, eliminating any clearance.

Hydraulic lifter preload is the distance the pushrod plunger is depressed below the retaining lock. Preload should be between .020"-.060" for the lifter to perform properly. With too little preload, the valve train will be noisy and the retaining lock may fail (break or pop out) due to excessive hydraulic force against the retainer. Conversely, too great a preload (more than .060") will produce excessive lifter pump-up, causing the valves to open longer and lift higher, drastically reducing cylinder pressure and hurting engine performance. The engine will also have very low vacuum, causing it to idle very poorly.

Checking and correcting preload - setting the valves - is relatively simple on engines fitted with adjustable rocker arms, like most Chevrolet and Pontiac engines. The traditional method involves checking each valve, one cylinder at a time.

Begin by checking the intake valve lifter preload as the exhaust valve has just opened. As the adjusting nut is slowly tightened, spin the corresponding pushrod between thumb and finger of the other hand. As soon as resistance is felt when spinning the pushrod, zero lash has been achieved. Further tightening, from one half to one full turn, will provide the correct amount of preload.

The exhaust valve lifter preload is checked next, just as the intake valve is about to close, and then adjusted in the same manner.

Many domestic V-8's are fitted with non-adjustable rocker arms. To check preload on one of these engines, remove the valve covers and loosen all the rocker arms.

Begin with the number one cylinder at top dead center (TDC) and lay a straight edge across the valve cover gasket surface of the cylinder head and scribe a mark on the each pushrod, right where the straight edge intersects it. Use of machinist's dye will allow you to easily scribe visible marks on the pushrods.

Next, tighten both rocker arms to OEM torque specifications and then scribe a second mark on the pushrods. This mark should be slightly above the first mark. Use a dial caliper to measure the distance between the two scribe marks to determine the existing preload.

If preload needs to be adjusted, there are several different avenues the engine builder can travel, each dependent upon what type of mounting the rocker arms use.

Probably one of the simplest ways to change preload is to change pushrods.

A longer pushrod increases preload and a shorter pushrod decreases preload. Adjustable pushrods are also available. Flat washers may be used to shim pedestal mount rocker arms like those used in many Ford, Oldsmobile, and American Motors engines. On Chrysler engines, and others with shaft mounted rocker arms, special spacers must be machined to fit between the cylinder head and the rocker shaft.

When should hydraulic lifter preload be checked? Something as simple as changing a camshaft or installing thinner (or thicker) head gaskets may affect lifter preload. Other engine machining operations such as valve grinding, decking the block, or milling the heads will, for sure, have a serious effect on preload. A good rule of thumb is that any time a cylinder head has work done, or has new components installed, is a good time to check and adjust preload.

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