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

ROLLGUY · #1 06-13-2015, 05:23 PM

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?

mach4 · #2 06-13-2015, 05:59 PM

I wonder if your replacement bearing was for an undersized crank, such that when you tightened up the other bearing caps even without the replacement one tight it was enough to bind the crank.

97 SL320 · #3 06-13-2015, 06:37 PM

It sounds like another bearing shell isn't properly located. Make sure the main bearing shells with holes are in the block and the other ones are in the cap.

Be sure to plasti gauge the main bearings also, you can lube later.

Did the crank measure OK? The rod knock after a bearing change is worry some. When plasti gauging rods, be sure to pull the cap off and not tap the rod away from the crank, this will give a false reading ( less clearance that you actually have. )

ROLLGUY · #4 06-13-2015, 06:57 PM

Quote:

Originally Posted by mach4

I wonder if your replacement bearing was for an undersized crank, such that when you tightened up the other bearing caps even without the replacement one tight it was enough to bind the crank.

Believe it or not, you may be correct. After pondering the situation, I went out into the garage and decided to completely remove both bearing halves and tighten up the rest of the caps. You guessed it, the crank spins freely. I only used one half of the bearing (harvested from an engine yesterday), but then used BOTH halves from the parts engine in my garage. Evidently the bearings I got yesterday were in fact undersized. Lesson learned: use the entire assembly, not just part!
Here is the other problem I am having with this engine. After sorting out the main problem, I plastiguaged all 5 rods, and they all are well within spec. The oil clearance is something like .39 to .79, and they all are around .51. I can't imagine what would have caused the rod knock with the new bearings. I showed one (the worst) to my engine guy, and he said they are fine. Is it possible that there could have been some bearing material on the crank the first time that would cause a knock? If so, it would have been polished off. I don't want to put the engine back together and find out the knock is still there. The car is a low mileage coupe with perfect body and interior. It would be a shame to put an engine back in with a knock.

mach4 · #5 06-13-2015, 07:10 PM

Is it possible the rod knock is coming from the other end - the wrist pin? Not likely, but who would have thought an engine would run nicely with 3 prechamber balls missing.

ROLLGUY · #6 06-13-2015, 07:18 PM

Quote:

Originally Posted by mach4

Is it possible the rod knock is coming from the other end - the wrist pin? Not likely, but who would have thought an engine would run nicely with 3 prechamber balls missing.

I know what that sounds like, and it is different. It is definitely a low knock. I am mad at myself for not loosening each of the injector lines to find out which one was noisy when I had it running. That would have made my troubleshooting a lot easier. I have to re-time the injection pump and put it all back together (including oil filter stand) in order to find out if the knock is still there.

97 SL320 · #7 06-13-2015, 08:53 PM

Quote:

Originally Posted by ROLLGUY

Believe it or not, you may be correct. After pondering the situation, I went out into the garage and decided to completely remove both bearing halves and tighten up the rest of the caps. You guessed it, the crank spins freely. I only used one half of the bearing (harvested from an engine yesterday), but then used BOTH halves from the parts engine in my garage. Evidently the bearings I got yesterday were in fact undersized. Lesson learned: use the entire assembly, not just part!

The real lesson is to read the back of the bearings and use plasti gauge every time.

Quote:

Originally Posted by ROLLGUY

Here is the other problem I am having with this engine. After sorting out the main problem, I plastiguaged all 5 rods, and they all are well within spec. The oil clearance is something like .39 to .79, and they all are around .51. I can't imagine what would have caused the rod knock with the new bearings. I showed one (the worst) to my engine guy, and he said they are fine. Is it possible that there could have been some bearing material on the crank the first time that would cause a knock? If so, it would have been polished off. I don't want to put the engine back together and find out the knock is still there. The car is a low mileage coupe with perfect body and interior. It would be a shame to put an engine back in with a knock.

Built up bearing material won't cause excessive clearance ( bearing knock )
You have other issues like a wrist pin or broken piston skirt.

You can check for loose pins by turning the engine so the piston is on it's way up, stop then push the piston from the bottom. If you feel any movement, the wrist pin bushing / pin is bad.

Stretch · #8 06-13-2015, 11:53 PM

Have you checked the crank end float?

ROLLGUY · #9 06-14-2015, 10:18 AM

Quote:

Originally Posted by 97 SL320

You can check for loose pins by turning the engine so the piston is on it's way up, stop then push the piston from the bottom. If you feel any movement, the wrist pin bushing / pin is bad.

As far as I can see, that is the only thing left that may be responsible for the knock. I have checked everything else.

ROLLGUY · #10 06-14-2015, 10:24 AM

Quote:

Originally Posted by Stretch

Have you checked the crank end float?

I have not actually checked it, but looking at the thrust bearings, they look like they were installed yesterday. In fact, the inside of the entire engine looks brand new. All the parts are clean as if they were just assembled from the factory. When I installed the crank, it seemed nice and tight with no give either left or right. It would be no big deal to fit a feeler gauge in there, but I am sure I will find that even the thinnest one I own may not fit (I will check the specs to make sure of the max clearance)....Rich

97 SL320 · #11 06-14-2015, 11:50 AM

I've never run into a situation where crank end play caused any noise.

If you check the wrist pins with the piston / rod out of the block, sliding the pin on the rod / piston can make it feel tight again and lead one to believe everything is OK. When an engine is assembled, the rod does not move much in relation to the piston so wear is concentrated over a small area. ( I'm speaking if crank guided rods not piston guided rods. )

As for being clean inside, diesels are like that. You'd think as sooty as the oil gets the inside would be worse than a gas engine.

ROLLGUY · #12 06-14-2015, 05:39 PM

Quote:

Originally Posted by 97 SL320

I've never run into a situation where crank end play caused any noise.

If you check the wrist pins with the piston / rod out of the block, sliding the pin on the rod / piston can make it feel tight again and lead one to believe everything is OK. When an engine is assembled, the rod does not move much in relation to the piston so wear is concentrated over a small area. ( I'm speaking if crank guided rods not piston guided rods. )

As for being clean inside, diesels are like that. You'd think as sooty as the oil gets the inside would be worse than a gas engine.

The pistons and head are still in/on the block. I am comparing the cleanliness to other 617 engines that I have gotten into, and this one is much cleaner. I will try to get a good photo of both....Rich

Stretch · #13 06-14-2015, 11:38 PM

Quote:

Originally Posted by 97 SL320

I've never run into a situation where crank end play caused any noise.

...

It can be a contributor - VW Beetle engines for example...

...but anyway for the context of this problem it doesn't sound too likely if the thrust bearings seem good

Dan Stokes · #14 06-15-2015, 08:10 AM

Remember that loose mains will cause a knock, too. It'll be lower in pitch - sort of a dull thud (at least in an Otto-cycle engine - I'd expect a Diesel to be similar). My $0.02 cents worth - if I was as far into an engine as you were I'd always do the mains. The cost of a bearing set vs. the labor is well worth it.

Those too-small bearings are pain! As someone mentioned, I always use Plastigage as a final check just to be sure all is OK. I've had "high-end" engine builders turn their nose up at Plastigage (I was interviewing for a job at the time) but it's the only technique that lets you measure everything in its "ready-to-run" condition, a great final check before you put on the pan. The advice to do it w/o lube was spot-on. Any lube has thickness and will throw off the measurement.

Another factor is the cleanliness UNDER the bearing though I'm sure you were appropriately careful about that. But some folks aren't!

Dan

ROLLGUY · #15 06-15-2015, 10:06 AM

Quote:

Originally Posted by Dan Stokes

Remember that loose mains will cause a knock, too. It'll be lower in pitch - sort of a dull thud (at least in an Otto-cycle engine - I'd expect a Diesel to be similar). My $0.02 cents worth - if I was as far into an engine as you were I'd always do the mains. The cost of a bearing set vs. the labor is well worth it.

Those too-small bearings are pain! As someone mentioned, I always use Plastigage as a final check just to be sure all is OK. I've had "high-end" engine builders turn their nose up at Plastigage (I was interviewing for a job at the time) but it's the only technique that lets you measure everything in its "ready-to-run" condition, a great final check before you put on the pan. The advice to do it w/o lube was spot-on. Any lube has thickness and will throw off the measurement.

Another factor is the cleanliness UNDER the bearing though I'm sure you were appropriately careful about that. But some folks aren't!

Dan

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. I would not have all the rod bolts (protected with rubber tubing of course) sticking up and getting in the way. I am about 99.9% sure the mains are good, and have no reason to suspect them being bad enough to make a noise that sounds like a rod, especially the way they look (barely worn, almost new looking). I will check for wrist pin play this afternoon after I get a Sanden retrofit done on a TD (it is summer here now!)....Rich