Timing-chain tensioner
Maintains suitable tension in the timing-chain path.
Beside a timing-chain guide inside or through the engine housing.
A spring and, on many designs, oil pressure load the guide to control slack and vibration.
An adjustable hand keeping a chain from going slack.
Start-up rattle may involve oil supply, the tensioner or guides rather than the chain alone.
Usually condition-based rather than a routine replacement. Lubrication, overheating and the exact engine affect wear.
Internal-engine work is usually labor-intensive. Diagnosis and the engine code determine the repair scope.
Replacing a BMW tensioner is not a substitute for checking worn guides or incorrect mechanical timing.
BMW petrol and diesel engines; petrol-only systems are identified individually.