Valve breakage at skirt end
Valve timing errors as the main cause of major engine damage
Damage to the valve control system is a challenge for any repair shop, as assembly or adjustment errors can quickly lead to major engine damage. A knocking noise, irregular engine running or the indication of a valve timing error in the fault code memory are sometimes early signs of a problem in the area of the cylinder head. Generally, however, valve breakage occurs suddenly and without previous signs.
Situation
A valve breakage in the area of the valve cotter halves occurs a short time after cylinder head repairs (Fig. 1). Valves with a skirt diameter of 7 mm or less are particularly affected. The defect does not give a warning, it occurs at high revolutions and often leads to major destruction in the combustion chamber and crank mechanism.
Cause of damage
The situation becomes particularly critical when the alignment between the crankshaft and the cam is no longer correct. An elongated timing chain or a defective camshaft adjuster change the valve timing so massively that the pistons in the bore knock against the intake or exhaust valves.
This mechanical contact causes enormous forces to be applied to the valve stem. As the valve in the guide has hardly any play, it experiences the highest bending moment at the bottom groove, bends at this point and finally breaks.
Overview of further reasons for failure:
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Mechanics: pre-damaged or bent tappets, rocker arms or finger type rockers, incorrectly mounted valve cotter halves.
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Wear: degraded valve guides, worn valve cotter halves or a weakened valve-spring.
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Setting: incorrectly set valve clearance or failure to observe the waiting time when installing new hydraulic tappets.
NOTE
A valve breakage on the skirt end can almost always be attributed to improper installation or unfavourable operating conditions. Material faults can be effectively ruled out as a cause. To prevent costly faults, the engine must be able to move absolutely freely by hand when turning manually before the first starting attempt.
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