There are some previous posts on this and they are the basis for my answer (I'm not a pro).

What I remember is, when you adjust the valves, you can check to see if you need a new chain by how many degrees of stretch are indicated. MB recommends 3 degrees is the limit but some of the pro's say 5 degrees.

I also seem to remember the rails usually do not need replacing. Howewer, if it were me, while your in there anyway, might as well replace them.

Anyhow, do a search or wait for someone who really knows what they are talking about to respond.

Regards,

Mr. BILL

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