Livin' For Lash
What separates a hydraulic lifter from a solid is the addition of an internal hydraulically operated plunger within the hydraulic lifter's body. The way the valvetrain is set up will depend upon whether the lifters are hydraulic or solid. Hydraulics generally run with pre-load, while solids must be set up with lash. Let's first consider a hydraulic. It's helpful to understand how the hydraulic mechanism works and what it does. Oil pressure enters the lifter through an orifice in the lifter body, and flows through another orifice into the hollow body of the lifter plunger. A one way check valve at the bottom of the plunger allows oil to fill the cavity below until all the valvetrain clearance is gone, effectuating the hydraulic self-adjustment to zero lash. When the cam rotates into the lift cycle, the check valve at the base of the plunger closes under the pressure imparted by the valve spring, preventing the oil from being squeezed back out as the valve opens. At the top of some hydraulic lifter plungers is a metering valve or plate, which supplies oil to the pushrods for valvetrain oiling.With the valvetrain installed (or adjusted), the pushrod compresses the plunger within its range of travel, and the hydraulic mechanism automatically zeros the lash. How far down the lifter plunger has been displaced at its base setting is called the lifter pre-load. The recommended pre-load with hydraulic lifters is usually in the range of 0.020 to 0.040 inch. Many stock valve-trains are non-adjustable, although many parts and processes involved in an engine build can alter the factory pre-load. In these cases, the solution is custom-length pushrods, or making the change to adjustable rockers.With adjustable rockers, setting the pre-load is simply a matter of setting the lobe being adjusted to the base circle, and tightening the adjuster until the clearance in the valvetrain is just taken out (zero clearance). Then turn the adjuster in 1/2 to 3/4 of a turn and lock the adjuster down.Solid lifters have no self-adjusting hydraulic mechanism, and need to run with clearance in the valvetrain. The lash specification is given on the cam card for a solid cam. The adjustment is made with the lobe being adjusted set on the base circle. A feeler gauge of a thickness matching the lash spec is inserted between the rocker and the valve tip, and then the adjuster is taken to zero lash and locked down. When the feeler gauge is removed, the lash will be set at the thickness of the feeler gauge. Lash is usually set cold when the engine is built, and then re-adjusted once the cam is run-in with the engine hot. The hot setting will be more true to the engine conditions in operation.