I wouldn't trust them that long... They are plastic (20 or so year old plastic @ that now) that has been heat cycled a multitude of times. I like to see the lower rails done around 300K.

You'll be pulling the front cover...which can, by some account, be done with the heads on but I'd recommend doing so while the heads are off for machining etc..

Also a great time to replace the water pump and the timing gears which are Sure to be worn @ 350K.

Jonathan

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Blue Ridge Mercedes
Jonathan Hodgman
http://www.blueridgemb.com/
Enthusiast Service, Restoration & Tuning.
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Located in the Atlanta area
Specializing in all pre and post merger AMG's including Hammers and DOHC M117 engines.
Mercedes Repair Atlanta

Replacing the lower right rail requires just about the same job whether done for replacement or after a failure. Back when we were converting lots of single row 380 chains to double row we did the job by removing the pan and pulling the front cover. This requires reusing the part of the head gasket that seals the top of the cover and caused a number of future leaks. We were much happier about the job when the chain system failed and we were able to remove the heads and do a complete job.

If your engine is virgin it has a better chance of having bad valve guides than it does having a bad lower rail. Do the upper three rails for maintenance and either throw it away or bite the bullet when the lower fails. or if you just wish to restore, do the whole job now from the top with a thorough valve job and a cover removal.

BTW: we charge 25 hours for a complete valve job and 40 with a front cover removal. Just to give you an idea of the work involved.

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