Camshaft followers are part of the valve actuating mechanism that contacts the
camshaft. You will hear them called valve tappets or valve lifters. The bottom surface is
hardened and machined to be compatible with the surface of the camshaft lobe. There are
two basic type of followersmechanical and hydraulic.
solid) tappets (fig. 3-47) are simply
barrel-shaped pieces of metal. When used in flathead engines, they have an adjusting screw
mechanism to set the clearance between the tappets and the valve stems. Mechanical tappets
may also come with a wider bottom surface. These are called mushroom tappets. Another
variation is the roller tappet. It has a roller contacting the camshaft and is used mostly
in heavy-duty applications.
are very popular in overhead valve engines. They use oil under pressure to maintain zero
clearance in the valve mechanism automatically. The lifter body, which contacts the
camshaft lobe, is hollow. Inside the lifter body, there is a plunger that operates the
valve mechanism. Injecting oil into the cavity under the plunger regulates its height,
thereby adjusting valve mechanism clearance. The hydraulic lifter operates as follows
(fig. 3-48): oil, supplied by the engine
lubrication system, reaches the lifter body and enters it through passage (1). The oil
then passes through passage (2) to fill the plunger. The oil then passes through passage
(3) where it pushes the check valve off its seat to enter the cavity under the plunger. As
oil fills the cavity, it pushes the plunger up to where it contacts the valve mechanism.
When the camshaft pushes the lifter body up, the oil is trapped in the cavity and cannot
escape because the check ball seals the opening. This trapped oil then becomes a solid
link between the lifter body and the plunger. The constant pressurized supply of oil will
maintain zero clearance in the valve mechanism.
The face of
the tappet and the lobe of the camshaft are designed so the tappet rotates during
operation (fig.3-49). The cam lobe is
machined with a slight taper that mates with a crowned tappet face. The camshaft lobe does
not meet the tappet in the center of its face. The design causes the tappet face to rotate
on the cam lobe, rather than slide. This greatly increases component life.
3-49.Tappet-to-can lobe relationship.