Solid/Mechanical Flat Tappet
The solid/mechanical tappet is essentially a solid “link” between the cam lobe and the pushrod. In most cases, it is a simple heat treated cylinder with a radius contact face. It allows more RPM potential than that of the hydraulic tappet since there are no worries about the inability of the lifter to “bleed down.” Solid lifters do, however, require lash or clearance adjustment in the rocker arm or push rod to allow for thermal expansion as the engine heats up.
Hydraulic Flat Tappet
Hydraulic flat tappets are self-adjusting due to a valve controlled plunger within the tappet body. It operates to pre-load the pushrod by using engine oil system pressure to maintain pre-load in the closed valve position. Hydraulic tappets are quieter than solid tappets since there is no lash or free-play. However, it is generally agreed that they fall short of offering optimum performance above 6000-6500 RPM. Many cheaper designs fall even shorter than this. This poor performance at high RPM is due mainly to the inability of the lifter to “bleed down” (leak down) the excess oil pressure and doesn’t allow the valves to seat.
The solid/mechanical roller tappet allows for the most aggressive lobe designs. Roller tappets allow faster, “steeper” opening and closing ramps. This allows the cam to produce more lift for a given duration. They are not limited to a particular lifter diameter to obtain higher cam lifts. They also contain a roller that reduces friction between the cam and followers. Roller cams require the use of higher valve spring forces, making high engine speeds (over 10,000 RPM) possible.
The hydraulic roller tappet camshaft can provide the best of both worlds. All Late Model engines have used this design since the mid to late 1980s. They provide most of the virtues of a solid/mechanical roller tappet while providing the benefits of quiet operation and ease of valve lash setting. This type of design still has the limitations of an oil bleed-off control type follower. If your application requires high RPM potential you should use a solid roller design.