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99 M3 S52 rod bearing 98k miles. It was a little rattly when warming for a few thousand miles but didn't sound like a bad rod bearing.This is the rod bearing on cylinder #6. At first it looked like the rod was bent. But the lower shell has a normal wear pattern. Talked with a friend that is pretty experienced and he and I think the shell might have warped or there was a small pressure drop out on a track day.

If the rod was bent it would show the same wear pattern top and bottom. We were thinking the shell got distorted and that's what caused the wear pattern.Luckily, the crank journal is in great condition. Looks like it wasn't far away from damage.

Any input would help. I'm going to install new bearings this week.

If you have tiny pits or pockmarks on the shell, it's cavitation erosion. Otherwise, it might be a bad shell.

If the head is off it would be a good idea to slide the piston out and have the big end of the rod resized. Otherwise, put the cap back on, measure the rod as closely as you can, and see if it's egged any.

I once saw an engine I knew for certain had never been tinkered with, that had a main cap installed backward from the factory. Plus a couple of others I was pretty sure had never been messed with, same way. If the rod cap will fit when turned the other way, it might have been that way when you took it apart. Mike it both ways and see if there's a difference.

The bottom of the lower shell has very fine pitting. They all looked like that. Seems like that was due to the car sitting after short drives, could be some cavitation erosion. but it is more consistent with chemical erosion.

Found the source of my bearing issue and noise/ vibration. Holy bent rod batman. It is may be a bit bent in the other direction.
90% sure it's due to hydrolock Only thing that can bend one rod like that and still run well.

Is that what it appears to be, the rod rubbing on the underside of the piston? Yikes!

[edit] Oh... bent rod, yeah that'll do it...

It was only making noise when warming, once fully warm it was pretty quiet. Still had odd vibes through the rev range.
Sourced a new(used) rod and slug. Going to pull the head and swap out #6, check the valves and put it back together.
only has about 98k miles and the inside of the engine is spotless.

Hydrolock can be caused by repeated attempts to start an engine with a leaking fuel injector.

It looked like the big end needed resizing to me too.

Picked up a good low mileage rod and piston as replacement. I'm going to have it checked, pull the head and swap out #6. #6 compression was a little low compared to the others, but leak down was good. the lowered piston height would account for that. Leakdown was very good they were all
around 205 psi with #6 at 188. Makes sense since the rod is a tad shorter now. Leakdown on #6 was very good.

You had better mike up the big end carefully, you'll get away with 0.0005" out of round/taper but anymore and you will need to start thinking about resizing.

I think you were lucky not to lose the engine if you were tracking it!

Looked at my notes again. Compression was 158 on cyl #6 not 188(my 5 looked like an . Finally removed the head.
# 6 sits a full 0.045" shorter maybe a bit more. Going to try and load some pics. Rod is a pretzel don't know how it ran well at all. Surprised it didn't go through the block!!!

Miced the journal with a good micrometer.. No taper or oval to at least 0.00025". Best I can measure.
Bore looks just like the others and measures well within spec. New piston and rod goes in with the old rings.
Glad the motor wasn't trashed. Looks like it has at least another 100k or more, wear is very slight.

Started cleaning up the block, head and pistons. Going slow since my 2.5 year old has been sick and the wife is VERY busy at work.
Head has been cleaned a little. # 6 was running richer than the others and had a lot of carbon deposits. Must be due to the lowered compression.
the bent rod was a full 1.7mm shorter when measured accurately. Did get a rod/piston, cleaned it, installed the original rings.

Plan on cleaning #3&4 tomorrow installing #6 and replacing the rod bearings

Could it be that #6 injector is crap, over fueling and leaking fuel when at rest, causing hydrolock?

Bill

Thought about that. I never had an issue with a hard or start/ stop condition. The starter doesn't usually bend rods but it's a possibility.
Five-O sells some disc style injectors from Ford motorsport that are a direct swap. They tend to give a little better fuel mileage and a couple of HP.
Under $300.00 for the set. Might just do that for some piece of mind.

Finished cleaning the block deck and pistons. Installed #6 with a very good used piston and rod. rings went on nice after a lot of cleaning. Slid right in with the help from the ring compressor and the wood end of my rubber mallet. Installed the new rod bearings with ARP bolts. Miced two with plastigauge, got about 0.0017 and 0.0016 ish. Piston #6 now sits 1.0mm proud of the block deck instead of below like before. Engine should be nice and smooth.