I think the replacement parts are brass. Also, the seals and seats for the injectors should be replaced while you're in there. I'd vote to replace them although I have not done that yet.

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Mark Herzig
1995 E300D 153K
1985 300D 142K (sold)
1979 450SL 122 miles (sold)
1992 500E 127K (sold)
1987 300SDL 132K (sold)
1986 300E 161K (sold)