I've never heard of an oil pump chain causing trouble on one of these engines until this thread. The timing chain is a piece of cake to change. You just have to remove the accelerator linkage, breather assembly, chain tensioner and valve cover. It takes a crimper to get the new chain linked but the old one is removed by grinding the crimped buttons off one of the links. Two, three hours start to finish maybe less if one has done it a few times and has competent help.

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84 300SD
85 380SE
83 528e
95 318ic