The OM-636.920 engine was used in the MB 180D and 180Db from 1953 to 1961. That was the lower trim-level version of the Ponton 190D, but the engines were somewhat different from the OM 621's used there. I would try the buy parts tab at the top of the page or call Phil. They will know if that water pump is still available and/or what else might fit.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150