The reason the rail breaks is not due to wear. The rail never touches the chain during normal operation.

The rail gets fragile due to temp, oil immersion, and age. The rail is there to keep the chain in line when the tentioner is not doing its job. NO MATTER HOW NEW, the tentioner will not do its job occasionally. The job it doesn't do is to check the oil pressure and maintain tention without pressure when the engine is off. The result is a momentary rattle on start up that instantly goes away when the oil pressure rises. The easiest way to see this is when the event randomly occurs after an oil change. When that happens the rattle is prolonged as the pump fills the filter. The oil change is not causing the problem it is just showing the relationship to pressure when one has lost it. If one wished to verify this (and had new rails to live through it), one could remove the tentioner and squeeze it in a vice. This will drive the retained oil from the device and when reinstalled the chain will rattle till pressure rises.

I do not change the tentioner unless the rattle event is common. If I hear it during service I will infer it happens regularly and suggest a tentioner. If you are aware and notice it when it occurs and it only occrs every few months or every few hundred starts I would not worry if I had GOOD rails.

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