I am having a pretty bad oil leak coming from a bolt that is between the fuel filter and valve cover, it is recessed and very tight. What does this go to? Sorry for the crappy photo

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In for answers too,my '81 300D does the same thing.. I have no idea where that bolt goes but I've thought about trying to tighten it to see if that'll help but I'm a little leery lol

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I've tried to tighten it, I've tried to loosen it and it won't budge

That part is the oil pump thrust piece (#124 in pic). This part is the top bearing of the oil pump drive shaft. It's a simple bronze bearing in an aluminum housing. The nut on top is a cap (122) that threads into the aluminum housing. Your leak is from an O-ring (120) that seals the housing to the block.

This part wears heavily (particularly on the OM617NA) and may need to be replaced. It can wear enough to allow the oil pump drive shaft to disengage from the pump. It isn't expensive.

When you remove it check the thrust face of the bronze bushing. When they wear heavily the oil pump shaft drive gear will cut right into the bronze.

To remove it you must remove a small bolt (123) that retains it through the side of the block. In theory it should just pull out but I have a drilled cap for each of the two sizes (early/late) for use with a slide hammer. You can also remove the cap and replace it with a bolt long enough to use with a lever.

Be sure to remove the cap and rig up a pulling method before removing the locking screw or the housing will just spin in the block when you try to remove the cap.

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1969 220D 5 Speed (OM616)
1983 240D 4 Speed
1985 300D Auto 376K
1985 300D Auto 275K

Wow thank you for such a informative post I really appreciate it and now my 34yr benz will stay on the road for many more years....even after I get back from deployment