Out of the hundreds of chain/timing failures my shop has done over the last 30 years only 2-3 were caused by the lower rail. When it wears through the chain rides on the steel retaining pins and the chain is too long to be tightened by the tentioner as a result. It probably made noise, but by the time I saw them they were broke. The results of the failure looked just like most of the others.

The upper rails are relatively simple to replace and are dirt cheap. They are good maintenance. I see no point in doing a job as maintenance that involves more than 70% of the cost of the repair it is intended to prevent.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician