My vehicle: 1985 300CD-T (Calif.).

Issue:
The two output seals that fit onto the sides of the differential don't seat entirely. Puzzling.

I've replaced the rear axles and differential seals on a 300CD before (but quite a few years ago now) and don't recall the output seals being so difficult to install and not seating entirely.

For each seal, about 1/3 of it sticks up (maybe two finger nail thicknesses) and doesn't seat perfectly flush like the other 2/3 of the seal.

Is this lack of complete seating likely a problem? Say, going to be the source of leaks?

The brand of the seal, incidentally is Corteco.

None of the McParts places seem to stock these seals (which strikes me as a bit unexpected), so next option would be replacements from the dealer.

Prior to installation, I applied 75W-90 oil onto the side of each seal as well as onto the differential surface where each seal would seat.

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I also just replaced the two drive line flex disks and intermediate bearing and carrier. I must say, the carrier, manufactured by Rein Automotive, was junk! Talk about not being anywhere near Mercedes-Benz quality! It seemed like the manufacturing had been pimped out to the super lowest bidder. Good heavens! It is as if the jigs for the parts are so old now that they are drifting out of specification...

The seals should seat the same all the way around. If not seated fully, indeed it could be the source of leaks.

I just did mine on my '85 300D and found a good bit of force to be necessary to get them fully seated. No leaks so far, about 2000 miles in. Corteco brand as well. They make the seals for Mercedes. You can see where they grind off the star before boxing them in Corteco boxes.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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On top of the seal, I've been using a piece of 3" ABS pipe cap and then a block of wood over that and a sledge for force. Or just the block of wood on the seal and the sledge.

I have pounded the stuffing out of myself but the seals just won't seat.

I wonder: If I used some MAPP gas on the inside of the differential where the seal isn't seating, if that would expand things just enough to get each seal to completely seat?

That did it:
Shot some MAPP gas on the opposite (inside) surface of where the seals were not seating completely. NOTE: Be careful with that MAPP gas stuff - it burns HOT, remember?
My technique: Use one hand to hold the torch while keeping a finger from the other hand on the inside of the seal. When you think that the seal has gotten warm enough, get the torch AWAY and let the whole ditty cool a few moments before returning to the block and sledge to drive the whole affair home.

Be careful of the Springs inside of the Seal on the Lip. I drove one of the Differential Seals in and the Spring poped off the Seal Lip.

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84 300D, 82 Volvo 244Gl Diesel