A tappet is a projection which imparts a linear motion to some other component within an assembly.
A tappet is only that part of a rocker arm which makes contact with an intake or exhaust valve stem above the cylinder head of an auto diesel.
as the cam rotates it creates both a sideways and a downward force would cause the valve stem to bend.
with a tappet the sideways force is transferred to the cylinder head so only the downward force acts on the valve stem.
Tappets are made from precision investment wax casting,chilled casting,forging material.the hardness given to all the tappets are above 58-60 so as to prevent the wear face wear out.