OM617 N/A main bearing problem- I'm baffled - Page 2 - PeachParts Mercedes-Benz Forum

pimpernell · #16 06-15-2015, 11:20 AM

Quote:

Originally Posted by ROLLGUY

A little background:
A friend brought me a low mile (less than 100,000) 617 that was going to go back into the car after being repaired. It was run low on oil and developed a rod knock. Both the customer and I thought (after inspection) that I would get away with just replacing the rod bearings. Since I have experience in changing the rear rope seal with the crank still in the engine, I was told to completely reseal the engine while it was apart. I replaced all the rod bearings, seals and gaskets, and put it back together. The crank journals looked acceptable, and no bearings spun. After setting it on the garage floor and hooking up all the necessary things to get it running (starter, battery, fuel, exhaust system), We were able to get it started. Almost right away the rod knock came back, although not near as bad as before (my friend drove the car before pulling the engine). We both decided that I should take it apart again, and inspect it. It appeared that #5 rod bearing had the slightest bit of scoring, but my engine guy (been in rebuilding business for 50 years) said that would not in any way cause a knock. He suggested I bring the crank in for polish, and then plastiguage each rod bearing afterwards. Fast forward to today: After picking up the crank, I lubed up all the main bearings and placed the crank in the engine. I was told to use the plastiguage dry, so i put no oil on the rod journals. I just hand tightened all the rod bolts so I could turn the crank slightly to get all the rods on. I lubed up all the main caps and tightened the bolts to spec (66 ft. lbs). I then went to turn the crank and it would not budge. I did notice that the 3rd main cap felt a little weird when I tightened it up, so I removed that cap. I noticed that the bearing was not seated correctly, and it messed up the locating tab a little. No need to worry, I had a parts engine just 3 feet away that I harvested a main bearing from. Just for grins, I left that cap off, and tried to turn the crank. It would not budge. I then loosened the other caps and with all the bolts loose, I was able to turn the crank. If I tightened only one of the other main caps, the crank was tight. I tried every combination, and the only main cap that is tightened to spec is the rear most one. If I tighten any other caps, the crank will not turn. I am baffled as to why this is happening. Anyone got any ideas?

I had purchased a 1990 Alpha Romeo Julia Spider that was not running, and the engine needed a rebuild. The engine had a knock, and the prior owner had just replaced all main and connecting rod bearings, but after about 100 miles, it started knocking again so he decided to sell it rather than spend more money on it. I had the crankshaft checked, and everything on that end was kosher, so I replaced all the main and rod bearings again. Started the engine and everything was fine for about 100 miles, then the knock came back.

Decided to have the connecting rods examined, and found that the number 4 connecting rod upper bearing housing was slightly bent, and the upper bearing was not sitting square on the crankshaft and would wear on one side, then after a few miles, would start to knock. The machine shop corrected the connecting rod flaw, and the car ran great from that time on. Good luck

97 SL320 · #17 06-15-2015, 05:22 PM

Quote:

Originally Posted by ROLLGUY

I would have liked to plastiguage the mains as well, but I did not want to put everything together dry, and then take it all apart to lube afterwards. If I were building the engine without the head on or pistons in the block, that would be different.

That's not an excuse , and your recent experience proves that. If the crank in the subject engine was cut undersized and a standard donor bearing was installed, it would be quiet at least early on , have low oil pressure at idle and , possibly crack the crank somewhere down the road due to it flexing.

Mains can be plastigauged then oiled with the crank in the block. After checking and cap is removed, dribble some oil down the sides / edge of the bearing then put some assembly lube on the lower bearing. Main bearing load is mostly down so the assembly lube will do just fine.

If there is still a concern about oil, some can be dribbled through the rod oiling drilling to the mains.

Quote:

Originally Posted by ROLLGUY

I would not have all the rod bolts (protected with rubber tubing of course) sticking up and getting in the way.

I'm not sure why you have rod bolts sticking up and no cap when working on mains. The crank does not get turned when plastigauge is on the bearing so even if the rod caps are off , there isn't an issue here.

97 SL320 · #18 06-15-2015, 05:25 PM

Quote:

Originally Posted by pimpernell

Decided to have the connecting rods examined, and found that the number 4 connecting rod upper bearing housing was slightly bent, and the upper bearing was not sitting square on the crankshaft and would wear on one side, then after a few miles, would start to knock. The machine shop corrected the connecting rod flaw, and the car ran great from that time on. Good luck

Plasti gauge should have found that problem as the entire rod would be bent and a full strip of plasti gauge would show taper.

ROLLGUY · #19 06-15-2015, 10:37 PM

As suggested, I checked the wrist pins today. I turned the crank so each piston was loaded against the crank, and then tried to push the piston down in the cylinder. There was no play at all in any of them, and I even went back and did it over again. I can say beyond any doubt that this engine has no wrist pin problems. I just don't know what else I can do or could have done to this one to fix it.
I once had an OM617 Turbo that had a knock. I pulled the head, and pushed down on all the pistons, and the front one went down in the bore what seemed like a quarter of an inch, making the knocking sound. I also noticed that the top of the piston and combustion chamber of the head looked polished, and no carbon was present. I assumed that the piston was hitting the head on it's rebound, and that is what was responsible for the knock. I suspected on the engine I am now working on, that if it did in fact have a bad wrist pin, It would have felt and sounded similar as the turbo engine that had the bad wrist pin. I now have no other tests that I can perform before putting the engine back together and testing it. As far as I can tell, there is no problem with any of the rods that might cause a knock. If it turns out that there is still a knock, the only thing that may make the noise is injector nailing. I had an engine once that sounded exactly like a rod knock, but after replacing the injectors, the sound went away.....Rich

Stretch · #20 06-16-2015, 03:58 AM

Ummm - I'd be checking piston protrusion before putting the head back on...

ROLLGUY · #21 06-16-2015, 10:26 AM

Quote:

Originally Posted by Stretch

Ummm - I'd be checking piston protrusion before putting the head back on...

That would mean that I would have to remove the head first. All the work I have done so far to this engine has been with the head on, and the pistons still in the block. The original job was to reseal the entire engine, and replace the rod bearings. Since I have had experience changing the rear rope seal without removing the crank, I was enlisted to do that job as well as the front seal and all the other gaskets. This engine is going back in a low mileage coupe that is undergoing a complete color change (that is a whole other discussion that does not need to be started

). I am out of options, and the only thing I can do now is put it back together and hope and pray it is fixed for good.....Rich

Stretch · #22 06-16-2015, 12:49 PM

Quote:

Originally Posted by ROLLGUY

That would mean that I would have to remove the head first. All the work I have done so far to this engine has been with the head on, and the pistons still in the block. The original job was to reseal the entire engine, and replace the rod bearings. Since I have had experience changing the rear rope seal without removing the crank, I was enlisted to do that job as well as the front seal and all the other gaskets. This engine is going back in a low mileage coupe that is undergoing a complete color change (that is a whole other discussion that does not need to be started

). I am out of options, and the only thing I can do now is put it back together and hope and pray it is fixed for good.....Rich

Ahhh - I was reading the bit you wrote about removing the head and I now see that was another engine - sorry.

I don't want to make work for you but a cross your legs and hope approach might not be the best now that the engine is out...

Dan Stokes · #23 06-16-2015, 02:49 PM

Hey Rich -

Best of luck on this!

Dan

ROLLGUY · #24 06-16-2015, 04:36 PM

Quote:

Originally Posted by Stretch

I don't want to make work for you but a cross your legs and hope approach might not be the best now that the engine is out...

What else is there to do? If I were building this engine from scratch, I would check using plastigauge the same way, and would have come up with the same results. For all intents and purposes, the rods check out well within spec. There is no wear on the thrust bearings, and the mains look very good (nothing like the rod bearings). The tests I did or the wrist pins check out positive (no detectable play). If I had an expert tell me of some other test or procedure to do before putting the pan back on, I would do it now. Knowing what I know, and seeing what I see, this engine should not knock when I get it put back together. Again, I don't know what else to do. The guy that enlisted me to do the work is in agreement with this, and I will get paid either way. I have done everything I was asked to do, and it was his call to put it back together. If it does turn out that the knock is still there, this engine will NOT go back in the car. He will have to find another one, and is prepared to do so if need be....Rich

mach4 · #25 06-16-2015, 05:05 PM

That being the case, I think you're doing the right thing...after all the customer is always right, and sometimes it just makes sense to cut your losses and move forward. Hopefully, the knock will be gone. And if not, it will be a shame to not know what the cause is.

ROLLGUY · #26 06-16-2015, 05:37 PM

Quote:

Originally Posted by mach4

That being the case, I think you're doing the right thing...after all the customer is always right, and sometimes it just makes sense to cut your losses and move forward. Hopefully, the knock will be gone. And if not, it will be a shame to not know what the cause is.

He told me that "it is made by man, therefore man should be able to fix it".
After consulting my local experts and the experts here, I have done all I can. Now I just need the motivation to put it back together!

97 SL320 · #27 06-16-2015, 06:04 PM

Quote:

Originally Posted by mach4

That being the case, I think you're doing the right thing...after all the customer is always right, and sometimes it just makes sense to cut your losses and move forward. Hopefully, the knock will be gone. And if not, it will be a shame to not know what the cause is.

No , the customer isn't always right. A customer seeks out the services of others because they can't do the job themselves and are relying on those with greater knowledge to help them. Letting them make poor choices is a disservice to the customer and unethical.

When I had my shop some wanted a patch that would not last, the conversation went like this.

Customer: I just want the car patched up by doing X Y Z , I'm getting rid of it.

Me: ( Thinking, no, you aren't getting rid of it after spending some $ to make it go. Or you will tell whomever the buys it that so and so just "rebuilt the entire car", here is the receipt. ) The repair you suggest isn't going to last.

C: A patch up is what I want.

Me: So you just want it patched up knowing it won't last, correct?

C: Yes

Me: So when it breaks, . . .I get to fix it the right way. . . for free. . .
~~~~~
Logical customers got it fixed the right way, the others when off to some low rent hack shop making my days much brighter.

Frank Reiner · #28 06-16-2015, 07:56 PM

Roller:

There is one (or two) additional possibilities; if a rod has a twist it will not be detected by plastigage as the centerline of the rod will not be out of square with the journal. Statically all will appear to be well, but dynamically a torsional oscillation will occur that will produce a knock. The second possibility is that there is a small fore-and-aft bend in the rod close to the pin end; piston clearance will allow for a slight tilt of the pin and piston in the bore. Again, all will appear OK statically, but dynamically an oscillation will cause a knock.

ROLLGUY · #29 06-17-2015, 12:49 AM

Quote:

Originally Posted by Frank Reiner

Roller:

There is one (or two) additional possibilities; if a rod has a twist it will not be detected by plastigage as the centerline of the rod will not be out of square with the journal. Statically all will appear to be well, but dynamically a torsional oscillation will occur that will produce a knock. The second possibility is that there is a small fore-and-aft bend in the rod close to the pin end; piston clearance will allow for a slight tilt of the pin and piston in the bore. Again, all will appear OK statically, but dynamically an oscillation will cause a knock.

I have never heard of either of these, and what would cause the rod to bend in such a way?