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Overhauling the valve system
251DOHC, AQ171
Technical Description
The engine has double overhead camshafts which
open and close the valves via hydraulic valve tap-
pets. The hydraulic valve tappets are filled with oil
and are self-adjusting. The valve guides have a 19°
angle to the cylinder head.
4 valves per cylinder
With two inlet and exhaust valves, the total valve
area has increased by approx. 50% compared with
the 230 and 250 (AQ131, AQ151). This improves the
gas flow to the combustion chamber and the fuel/air
mixture is burnt more effectively.
The camshafts are fitted in a special camshaft carri-
er fitted on the cylinder head. The seal between the
camshaft carrier and the cylinder head is done in
two ways. The spark plug wells are sealed by four
O-rings between the camshaft carrier and cylinder
head. The sealing of the flat surfaces between the
camcylinder head is done using a sealing compound
(Loctite).
The valve cover gasket is in two parts. One seals
against the spark plug wells while the second part
seals between the camshaft carrier and valve cover
outer boarder.
Hydraulic valve tappets
Between the camshaft and the hydraulic valve tap-
pet there is normally a certain clearance. The clear-
ance is greater when the engine is cold than when it
is hot to allow for the lengthwise expansion of the
Piston
Groove
Check valve
Spring
Tappet cylinder
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