I used a metal oil cap to replace my plastic cap on my 617 Turbo engine, and despite having a new, correct seal it still leaks oil. I also noticed that I can slide it side to side 1/4" or so when it is on and tightened down.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine