The essential difference between solid and hydraulic (fluid operated) lifters is that hydraulic lifters contain a hydraulic plunger within the lifter body. Other differences between the two types of lift affect the way in which the valvetrain in an engine is configured and engine performance.
Typically, solid lifters must be set up with lash, or mechanical clearance between the cam lobe and the valve stem. Hydraulic lifters, on the other hand, require preload, or distance between the retaining snap ring and the plunger seat when the valve is closed.
Hydraulic lifters feature a self-adjusting hydraulic mechanism. Oil under pressure flows into the lifter through one opening and enters the hollow body of the plunger through another. Solid lifters contain no such mechanism and are typically set when the engine is built and readjusted later.
Solid lifters provide better control of the valvetrain as engine speed increases and reduce a phenomenon known as deflection, allowing the valvetrain to produce more revolutions per minute (rpm). Solid lifters provide better performance, especially at 7,000 rpm or more.
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