Engine Overhaul Manual v8 4.0 4.6
ENGINE
8
DESCRIPTION AND OPERATION
Hydraulic tappets
1. Clip
2. Pushrod seat
3. Inner sleeve
4. Upper chamber
5. Non-return ball valve
6. Spring
7. Outer sleeve
8. Lower chamber
The purpose of the hydraulic tappet is to provide
maintenance free and quiet operation of valves. It
achieves this by utilising engine oil pressure to
eliminate the mechanical clearance between the
rockers and the valve stems.
During normal operation, engine oil pressure,
present in the upper chamber, passes through the
non-return ball valve and into the lower chamber.
When the cam begins to lift the outer sleeve, the
resistance of the valve spring, felt through the push
rod and seat, causes the tappet inner sleeve to
move downwards inside the outer sleeve. This
downward movement of the inner sleeve closes the
ball valve and increases the pressure in the lower
chamber sufficiently to ensure that the push rod
opens the valve fully.
As the tappet moves off the peak of the cam, the ball
valve opens to equalise the pressure in both
chambers which ensures the valve closes when the
tappet is on the back of the cam.
