They are replaceable, but must be removed and replaced at a machine shop, or a shop that has access to a machine shop. Not a DIY project. I just got an estimate for that, myself, although mine aren't bad enough yet to require it.

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002