Worst possible failure the 1978 300sd I bought for my son just had a catastrophic failure of the rear main seal! We confirmed this with dye test looking thru access opening on transmission. It went from a few drops a day to a quart every 8 or 9 miles. We have never attempted a repair this big our selves. Motor has compression all right around 350 to 375 200k on the odometer. Not sure it is worth the huge effort to repair if it is the upper half of the seal that requires the crank to be removed. One we never have done such an extensive repair it may be above our ability. Also cost with new bearings and bolts, I have not priced them yet but when you add to that special tool required a torquing angle set? Do you really need one? Can it be rented? I guess what we will do is pull the motor drop the upper pan and hope there is a large chunk of seal missing so we could just replace that half of the seal. All in all very discouraging! The only good news is we bought a hard copy service manual off ebay. If it was only as easy as reading the manual makes it sound.
Dave

It is a good diy'er type of job. Do you have an engine lift/stand? If it were me I would remove the crank shaft and go ahead and replace the upper/lower half of the seal. Go ahead and replace the rod/crank brgs too. If possible, leave the pistons in the holes (with that good of compression). Plan on replacing the oil pump chain guides while you are in there. The biggest problem with this type of job is the "may as well disease". Once you get one torn down to that layer you get to the point where you say to yourself I may as well replace ________ while we are in there. That is when the costs add up.

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Jim

Jim,

I am going to suffer a case of "may as well" disease in a short while. The SD needs a new head gasket. I am tempted to just do the gasket because the engine runs perfectly right now. But, Greg, dammit, in his infinite wisdom, gave me a case of the "may as well" disease with regard to manifold studs, valve springs, valve seats, valve seals, and valve guides.

Since I now have the disease, maybe you too could offer your insight as to anything else that will certainly make the disease WORSE

Manifold studs and valve springs would not be on my short list both chains and all chain guides would be along with new tensioner and tensioner spring. Take a good look at the oil pump chain - it will outlast the timing chain BUT if you are 400k+ miles I'd do it and the oil pump for the heckuvit. Inspect
all sprockets real good and make sure you don't detect any looseness indicating a wallored out keyway on the crank.

Minimum stuff would be oil filter housing gasket and fuel injection pump gasket along with new gaskets at the fuel pump. I would look to see if there were any cross hatching in the upper parts of the cylinders to determine if the bores needed some attention. If they look like mirrors install new sleeves and rings and hope that the ring lands are not damaged on the pistons.

good luck

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Jim

Well I did a butcher job on a core motor I have. I removed the timing chain tensioner and pulled the upper oil pan, oil pump and crankshaft bearings. I was able too lower the crank enough to get the seal out, it flaked out in chunks. That killed any hopes of cutting the oil pan seal a little long so it would bunch up the upper part of the seal. I wish I had tried to just loosen the crank bearing enough to see if I could change it with out taking the bearings out all the way. It was not that hard but then I just had a block and head that I did not care about. But now I know I can do it without pulling the piston bearings well I think I am safe that way. Cost is a huge consideration I got the deal of a lifetime on the car $50. But my son is in collage and works part time and I am on long term workers compensation and in California that’s not 5 percent of what I made at work so money is real tight. And now we are down to one running car. The good news is we have all the tools except the special torquing angle set I hope we can rent it or get by with out it. If we could do it for under $125 we might as well try. But god when we are done if it leaks when I am done crying I will have the best amusement park ride of my life when I take it on the fire roads and see how much punishment a 116 can really take. Well for various reasons we can’t start till the beginning of the month. I guess all we have too loose is about 60 hours of work mostly mine and a lot of pain. I must thank you all because without this board I would never even think of trying this. My plan is go by the book go slow make it a bunch of small jobs. Any info about the torquing angle set any at all would be appreciated.
Dave

I got lucky used a bottle of stop leack that freshens up seals leak is a drop a day for the last 700 miles I just hope my luck holds.

if you are replacing the rear seal I hope you do the front one too at the same time. its likely about to start leeking too if it isn't already, and like you said dropping the cranks is such a PITA since you have to pull a motor to do it.

My front seal is leaking , and likely the rear one. not enough to matter but enough to tick off people for leaving oil drips on their driveways.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche