For some reason there are no published specs for the torque.

http://www.peachparts.com/shopforum/showthread.php?t=63971&highlight=manifold+torque

I just used firm pressure on a standard ratchet.

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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