Is there an easy way to know how much abradable material is sufficient? Is it possible to inspect and assess it while still in the vehicle? I haven't tried, but physical access and visibility must be poor while it is still in the engine. Thanks.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

It's possible to the make a determination once the tensioner is removed. That side of the chain will be very slack and you can pull the chain off the tensioning rail to take a peek.

You'll be able to see the edge of the abradable material and make a judgment on how much has worn on the face of the rail.