Valve engine problem Brisbane - BENT /

BROKEN

Bent valves are usually the result of a collision between the piston and the valves. In DOHC engines the collision can occur between the inlet and exhaust valves. The most common cause of the valve making contact with the piston is an interruption to the cam shaft timing. This is usually caused by a timing belt or timing chain failure. The photo is an actual instant where a timing belt failed due to the belt adjuster lock nut not being tensioned correctly at a retail service centre. The investigation and insurance repair was given to us and we obtained these photos as part of our report. The belt ran very loose for a considerable period stressing every tooth on the timing belt. Finally the belt stripped and all the exhaust valves contacted the four pistons and were bent and snapped at the stem. Some manufacturers by design make their engines what we call "a safe engine", which means even with a broken timing belt the valves don't contact the pistons. Unfortunately this was not one of these. Another cause is valve bounce during over revving. This can cause contact of the valve to the piston and bend the valve. Even without piston to valve contact the stress from continued valve bounce can cause the valve head to fracture at the neck of the valve. Excessive tappet clearance will cause the valve to contact the seat at a higher speed which can also stress the valve head and stem to breaking point. (the valves contact the seats while further up the lobe profile rather than from the closing ramp.) The induction of foreign material is a more uncommon cause. Disclaimer Agreement : Every care has been taken in writing this information and procedures, but no responsibility can be excepted for errors, omissions or misuse of this information and procedures. The information available on this site is for your instruction only and cannot be copied for sale, © copyright 2001 UMR Engines