Timing Belt and Timing Chain replacement is part of your maintenance. As the belt/chain ages it wears and reaches a point of failure eventually.
Timing Belts are made from the same basic material your drive belts are made from. As the rubber and material ages out dries out and begins to crack weakening the belt. Most belts are recommended to be replaced between 70-100,000 miles. Check your owners manual for specific intervals.
Timing Chains are designed to last longer than traditional belts. They can still fail and even wear prematurely. Oil quality plays a huge role in timing chain wear. This is why oil changes should always be performed at the recommended times. The other reason for chain failure could be broken or worn chain guides. Most guides are made from plastic and weaken over time due to exposure to extreme heat causing them to break.
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