Rear main seal
 

I'm about to pull the engine on my 300SD, due to a bad oil leak. It looks like a rear main seal to me. However I herd somewhere these rear main (rope seals) almost never go bad. Anyone have any experience with this.

It can't last ForEver.....
They wear out just like other seals.... we are just so used to our engines going much longer than usual engines that we are surprised when something like this gives way....
However, if you want to hold on to hope.... I am assuming you are thinking it is leaking from the placement of the drip...
It could be the big ball bearing placed at the end of the main oil channel ...
But you have to do most of the same labor to get to that anyway....

Hey, just saw your location....another candidate for Central Texas Gettogethers....

Verify its not your headgasket seeping oil at the rear of the head, thats far more common than rear main seals leaking.

I was hopeing some one would come up with the ball bearing theory.

Hope Springs Eternal.....

I replaced the head gasket a couple of years ago. My dad and I took it to the car wash and blasted all the oil and grime off of it. We cleaned all around the oil filter and back of the head good. When we drove it home and looked there was only oil coming from the split line between the engine and transmission.
I sure don't want to pull that crank :(

Sure don't blame you there... and if you plan on doing it during Spring...you had better get crackin.... only about a week left of that...and it will be getting hotter....

main seal
 

be sure while you are at it to check the bearings for wear. often if the bearings are shot the seals wont hold the pressure which is greater if the bearings are loose.

I can't seem to find a primary oil pan gasket for a 617. I found a vague reference to sealing compound in the factory manual. Is this one of those gasket in a tube things?

No gasket on the upper aluminum oil pan - just the smaller, lower one.

I got the engine/transmission pulled this weekend. I can't see where the oil is coming from, but I suspect the two interference fit ball bearings that cap off the oil passages. Next time I have off I will clean it up real good, fill it with oil, and roll it over with the starter to bring up the oil pressure. If these passages leak I will replace them with two 1/2 inch pipe plugs.

I still have not seen anyone give a logical explanation as to why those balls are there instead of a pipe plug... in the first place...

The only thing I can figure is, it's cheaper to drill a hole and punch a ball bearing in it then it is to drill and tap (two steps) for a plug. Witch also requires machine work. If the same method were used as for the front, it would also require facing for the copper washer to seat to. I thought the same thing and this was the only explaition I could come up with.

I got the rear main replaced. I had to pull the crank! I also replaced the two ball bearing that close the oil passages in the back with 1/2 inch pipe plugs. This should fix my oil leak. I got the engine all back together and ready to drop in. I did take the opportunity to check the B2 piston and thank goodness I have the upgraded piston. I tore the engine down so far I might as well overhauled it. I should have it up an running by Wednesday. I'll let you know how it turns out

Any pictures of while you were doing it all?

No I didn't get any pictures. I thought about it, but I didn't want to do that to my digital camera. There was a constant 1/8 inch thick black coating on my hands. It was loosing a quart every 500 miles. I didn't want ya'll to see my crooked tap job either. They aren't pretty but they are sealed.

I finished installing my engine yesterday. The leak from the rear of the engine was the same, possibly worse. I'm going to do the dye in the oil thing and pull the tranny. Anybody got any suggestions? It leaks about one drop every second.

The Leak is Fixed
 

I borrowed a UV light from a mechanic friend and bought some oil dye. My friend told me to use two bottles, but I only used one. I should have use two like he said. I had some difficulty seeing the dye. I tracked it down to the lower gallery that feeds the main bearing. I pulled the plug and wrapped teflon tape around it (4 turns) and tightened it down. I then put the backing plate and flywheel on and I put two bolts to hold the starter on and started it up. It really was not that loud. I let it idle about 5 minutes and no leak. :)

I had a leaking valve cover gasket that only leaked in the back. I would have bet my life it was the rear main seal because I had just replaced the one in the front. It was by pure mistake I noticed it.

Frank, can you expand on the B2 piston issue? I was planning on replacing my rear main seal this weekend, but after reading this thread, I'm not convinced that's where the problem lies... I've already replaced the pan gasket, valve cover gasket, and the two gaskets on the oil filter assembly. I keep seeing oil in the seem between the engine and transmission.

Does the crank have to be pulled to replace the seal? Does the engine? I was planning on just pulling the transmission...

Any help appreciated!

-Colin

If it is the 616 or 617 diesel engine they yest to both. Pulling the tranny won't allow replacement of the rear main on these engines.