Figure 3-55. Mechanical Tappets.
plunger will regulate its height, thereby adjusting valve
type of design causes the tappet face to roll and rotate
mechanism clearance. The hydraulic tap-pet operates as
on the cam lobe rather than slide. This greatly increases
follows: oil, supplied by the engine lubrication system,
reaches the lifter body and enters it through passage (1).
3-14. Valve Operating Mechanism.
The oil then passes through passage (2), to fill the
plunger. The oil then passes through passage (3),
a. General. The valves in overhead valve and
where it pushes the check valve off of its seat to enter
overhead camshaft engines use additional components
the cavity under the plunger. As the oil fills the cavity, it
to link the camshaft to the valves. Overhead valve
pushes the plunger up to where it contacts the valve
engines use push rods and rocker arms. Overhead
mechanism. When the camshaft pushes the lifter body
camshaft engines use various configurations of rocker
up, the oil is trapped in the cavity and cannot escape
because the check ball seals the opening. This trapped
b. Push Rods (Fig. 3-58). Push rods usually are
oil then becomes a solid link between the lifter body and
constructed of hollow steel. Most air-cooled engines use
the plunger. The constant pressurized supply of oil will
the push rods to supply lubricant to the upper valve
maintain zero clearance in the valve mechanism.
c. Rocker Arms (Fig. 3-59). Rocker arms are
i. Camshaft-to-Tappet Relationship (Fig.3-57). The
manufactured of steel, aluminum, or cast iron. The most
face of the tappet and the lobe of the camshaft are
common for current use are the stamped steel variety.
designed so that the tappet will be made to rotate during
They are lightweight, strong, and cheap to manufacture.
operation. The cam lobe is machined with a slight taper
They usually pivot on a stud and ball, though some
that mates with a crowned lifter face. The camshaft lobe
engines use a shaft arrangement. Cast iron rockers are
does not meet the tappet in the center of its face. Using