Just replaced the oil pan and gasket on my 84 300sd but its leaking! I made sure the surfaces were super clean and tightened the bolts in a criss-cross pattern.... What gives!?!?!

Should I scrap the gasket and go with one of those 3M sealants from Autozone??

Any advice would be greatly appreciated!

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Assuming that you are referring to the lower oil pan, make sure that it is actually the pan that is leaking. There are plenty of things above it that can leak and give the appearance that the pan gasket is the culprit.

I would not go with the 3m sealants by themselves... ie, intead of the gasket .. if that is what you mean .
What was the gasket made out of ?
IF cork I always COAT the gasket with the 3m sealant... all sides... which means doing one surface at a time and letting it cure before doing the rest. This really gives a great gasket.
Even with rubber gaskets I glue them to the part which is movable ... that means NOT the engine block... the pan or the valve cover gets the gasket glued to it...
I use what is called ' yellow death' ... a search will bring up other threads about this topic... then when putting together always coat the surfaces with clean oil if they are oil items.. ( no oil on paper water pump gaskets for instance )....
It sounds like you put a new pan on (?)... all pans should be checked for flatness at the sealing surface... not always easy to do...
The most common cause of gasket failure is over torquing of bolts... or not using the cross pattern tightening.... also run a tap into all the holes to be sure they are clean.. any little item in the threads messes up the torquing accuracy.
Naturally all the bolts need to be clean ...

Hi there I wouldn't touch the RTV sealant with a ten foot pole.Its just not needed.Its critical that the surfaces are clean and mate properly but its even more critical that the bolts be tighten according to specs.They should be no more then 2.4 Ft Lbs each.I have done mine just recently and I have it on my website witch I will give you the link...You will also see the effects of RTV sealant that the previous owner applied.MB does not call for sealant as long as both surfaces are clean and that the torque is respected.
Now on the other hand,if you did torque them to spec,your probably looking at re threading the oil pan casting with Helicoils,another easy job.
Here are both links...first the oil pan
http://benzguy.piczo.com/oilpanturbooilreturntubegrometandsealreplacement.?cr=7&linkvar=000044
and now the re threading with Helicoil.
http://benzguy.piczo.com/howtodothreadrepairsonTransmusinghelicoil?cr=7

Good luck
Big Dan
Vancouver Canada Eh!

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Big Dan MEng

Visit my Blog..
http://allformercedesbenz.blogspot.com/

www.benzguy.piczo.com



If at first you don't succeed,
Skydiving is not for you.

ARTIFICIAL INTELLIGENCE IS NO
MATCH FOR NATURAL STUPIDITY.

If anything can go wrong,fix it!
The hell with Murphy!!
I never think of the future.It comes soon enough.

I'm likely to need to do a helicoil for one of my lower oil pan screws. A head broke off when trying to remove the screws to put in a new gasket.
Any idea on the screw size?

'85 W123 300D

No I'm sorry Matt i don't know,but in any case you would be better off doing the Helicoil on all 19 bolts.I know it looks like allot to do but when you buy a Heicoil kit it probably comes with about 15 to 20 inserts.So its all up to you but if it was me....I would do them all.Not difficult at all. And will probably take you about 2 hours to do.
Good luck
Big Dan

__________________
Cheers
Big Dan MEng

Visit my Blog..
http://allformercedesbenz.blogspot.com/

www.benzguy.piczo.com



If at first you don't succeed,
Skydiving is not for you.

ARTIFICIAL INTELLIGENCE IS NO
MATCH FOR NATURAL STUPIDITY.

If anything can go wrong,fix it!
The hell with Murphy!!
I never think of the future.It comes soon enough.

thanks.

You do not want a criss-cross pattern. imagine sqeezing the gasket out with the screws. Start in the center of the pan on each side. Snug one on the front, snug one on the back, and squeeze the gasketout, away from the center.

then do a final torque once they are all snugged

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LOL, ' criss-cross' is a pretty loose concept...... seems like you are in tune with it in general except that you are specifying starting in the center and spreading out and around... which is also the specified pattern on tightening the head bolts...
Lets add that all the torque is not applied at one time to any bolt... we are trying to squeeze it up even and level....

That happened to me. I would have sworn that the seal was leaking, but in fact the oil that leaked from above collected around the seal.