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The valve lifter is the unit that makes contact with the valve stem
and the camshaft. It rides on the camshaft. When the cam lobes push
it upwards, it opens the valve.
The engine oil comes into the lifter body under pressure. It passes
through a little opening at the bottom of an inner piston to a cavity
underneath the piston. The oil forces the piston upward until it
contacts the push rod. When the cam raises the valve lifter, the
pressure is placed on the inner piston which tries to push the oil
back through the little opening. It can't do this, because the
opening is sealed by a small check valve.
When the cam goes upward, the lifter solidifies and lifts the valve.
Then, when the cam goes down, the lifter is pushed down by the push
rod. It adjusts automatically to remove clearances.
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