Shot Crankshaft Bearing - Please Help Identify the Issue

[Pawel66]

What you see on the picture is shot crankshaft bearing No.3 (thrust bearing) on M130 engine (1971 280SL, US) that made 550km after overhaul where all the bearings, pistons, rings, etc. were replaced with new ones. The overhaul was necessary after catastrophic failure when oil hose blew open and the car lost oil. Crankshaft bearings 4 and 5 were affected during that event, engine stopped. My friend could not reach the oil pressure level from before the event, decided to pull out engine again and found this bearing.

All other bearings on crankshaft and cranks look like new.

The key question from my friend to this group is: if the crankshaft was bent during the event, then was straightened for machining and machined - can it bent back again when working?

The crankshaft is now bent 0,07mm. We do not know if it was bent after the event, the shop did not make a record. In some engines straightening and machining works just fine, in some other engines it does not - crankshaft bends again. it was straight during the assembly - moving freely by hand as it should. It could not be machined if it were bent. Does the crankshaft bend back in M130 after straightening, when it is fitted and starts working?

This is our working theory now.

We do not think it is lubrication - the crank bearing for piston no. 3 is lubricated through this bearing in question and the crank bearing looks like new.

Other potential issues? Material?

It is most likely not the question of sizes and measurements.

Please help identify the issue.

[Benz Dr.]

I've seen where bearing shells were installed with the oil galley hole on the bottom retainer instead of in the block. Whatever happened, that bearing was really tight or it ran without lubrication. 

When an engine seizes up the kinetic energy in the spinning crank will literally twist it as it comes to a stop. The quicker it stops, the more it will bend. Metal has elastic memory and somehow I think bent cranks should remain that way. At 550 miles on a fresh engine, something went badly wrong.

[Benz Dr.]

One other thing. When an engine fails it often leaves small shards of metal throughout the entire oiling system. Oil coolers are very good at trapping these pieces and then releasing them into the oil stream. Make sure everything has been cleaned well before assembly.

[Pawel66]

Benz Dr., thank you for an answer.

They ruled out the lubrication as, as they said, the crank bearing for the 3rd piston is lubricated through this no. 3 bearing of crankshaft and the crank bearing was fully intact. I am enclosing lubrication diagram. Again - all other bearings are perfectly intact...

That is why they had a theory that the crankshaft bent when the catastrophic event occurred, then it was straightened for machining, but it came back to be bent when it started working.

But I see you are more inclined to say it was lubrication or tightness issue...

Well, it was even 550km, not miles - until the first oil filter (the run-in one) was exchanged. They saw oil pressure was lower than before overhaul. They thought it might have been because of the more tight oil filtration. They made 500km, changed the filter to a regular one and they saw no improvement in oil pressure. Pulled out engine again, disassembled and saw the bearing as on the picture.

[Cees Klumper]

If I understand correctly, the oil pressure was relatively low right from the beginning and did nog improve during those 550 kms? If it was so low from the start, that would point to a problem right from the start.
Maybe an obstruction in the oil passage that feeds this bearing, or the crankshaft did 'bend back' right after machining as you speculate. Because as you state it is bent again now, but there was no catastrophic failure this time, only low oil pressure.
Or - crank was straight when fitted, but poor lubrication to that one bearing, it failed, put pressure again on the crankshaft in that one place, which caused the crankshaft to once again twist?
If you need any used engine parts to replace what's failed, I have them in France and I will be there for a week or two in February.

[Pawel66]

Cees, thank you for an answer.

Lubrication issue is first that comes to mind. But there is this crank bearing in mint condition fed oil from the failed bearing, which suggests no obstruction in oil passage. That is why we are speculating - bending first, bearing failure next rather than oil issue, bearing failure then bent crankshaft.

I will check if this lower oil pressure (my friend was looking for 0.5 bar as it was that much lower than what he had before, but still within specs) occurred from the very beginning or came after some time. As you say - it may play a role, it had to take the crankshaft some time to bend back again.

Seems to me, but I am not sure, that since you are not immediately saying "you cannot straighten crankshaft in M130 and fit it again as it will bent back anyway". If it were the case, you would have known it very well.

[Harry]

I would lean towards the crankshaft being a consequence and not a contributor.  Low oil pressure after rebuild is a sign that something isn't right.  It should be at its best at that point.  Unfortunately at this juncture, the only option is to start again, being attentive to potential issues during the rebuild.  This can include being certain that the oil channels in the block are super clean and clear, being to ensure installing the main bearing in the correct orientation (such that the lubrication hole is located correctly), being sure the oil pump itself is in good condition, replacing the oil pressure relief valve, using an assembly pre-lube liberally, confirming good rotation of the crank when installed and without connecting rods attached (smooth with no hard points).  And of course, recheck all other bearings, including the camshaft, pistons, rings and bores, etc.  Everything should be suspect to suffering damage and needs to be confirmed to be OK.
Sorry to hear this though.  I know it's disappointing but I'm sure you'll get to the bottom of it.
Harry

[Pawel66]

Thank you for your answer!

[Benz Dr.]

Would you be able to get a picture of the crankshaft? In particular, the thrust bearing journal.

[Pawel66]

Just this one at hand for now...

[Pawel66]

The relatively low oil pressure was there from the first start - after engine warmed up to operating temperature.

[stickandrudderman]

I don't believe that bearing has suffered oil starvation; if it had it would be blued from the heat. I suspect a lack of clearance.
Did whoever built the engine use plastiguage to check clearances during assembly? I always do and in the past have spotted an excessive bearing clearance that turned out to be a problem with after-market shells. Genuine MB shells gave the correct clearance.

[Pawel66]

Thank you for your kind attention and tip.

The plastic strips tests were done.

I told my friend - perhaps the bearings were not ok. The shop brought them from Germany, not sure what the brand was. I will pass on the recommendation to use MB bearings, if you say that, it is important.

[Benz Dr.]

Crank was running hot but I agree that it looks like a clearance problem. Block may need an align boring as the bearing caps may be out of round.

[Pawel66]

By clearance problem, I understand, you mean bearings being too tight? Or their seats in the block not aligned?

[Benz Dr.]

By clearance problem, I understand, you mean bearings being too tight? Or their seats in the block not aligned?

The main bearing cap may be slightly out of round. This will cause the bearing to squeeze in that area and quickly wear through the plating on the bearing shell. Since it's not a rod bearing, odds are you may not hear anything that would indicate a problem.

This is why you always resize rod bearings -they often go out of round. Main bearings - not so much.

[Pawel66]

Thank you!

[stickandrudderman]

Crank was running hot but I agree that it looks like a clearance problem. Block may need an align boring as the bearing caps may be out of round.

Good call. I second that, get the block line-bored.

[Pawel66]

Thank you both!

[ja17]

Yep make sure all the bearing caps went to the correct location.  Also, as I recall there are two style of blocks with two different bearing sets. Make sure you used the correct bearing set. Once you have a tight spot, or foreign debris, or a mis-alignment, or lack of lubrication, heat will build up quickly and ruin the bearing. Often it will then weld itself to the spinning crankshaft and spin in it's housing (spun bearing). At this point you will most likely need to have the block align bored, the crankshaft checked and reground. Make sure the correct bearing set is ordered by the engine number. Engine break in is critical. The first dozen or so starts should be short runs, then cool down and restart and run a little longer each time. Allowing those hot spots in the engine, during break in, to normalize before they overheat and cause failure. "Stick" is correct, about contamination. Just one tiny piece of metal in some dark crevice, can become loose and ruin the job. The internal oil galleries in the block have plugs that can be removed to clean all these passages. Also the oil cooler and lines must also be flushed! At this point, metal particles are everywhere.

[Pawel66]

Than you Joe, appreciate your advise! The engine is very late (71) US engine. We will make sure the next bearing set is correct.

[Pawel66]

This engine is fixed now, just wanted to close the topic.

Cleaned, realigned, rebalanced, set of bearings bought from Mercedes, assembled and 500km further everything is neat, clean and quiet with oil pressure as before the catastrophe of shot oil line.

Apparently it was the main bearing fitted too tight.

Many thanks to all contributors!

[Cees Klumper]

Thanks for posting the final outcome Pawell, much appreciated.