Destroyed m20 Rod Bearings

[duggi]

Anyone want to know what fully-spent, m20b27 rod bearings look like after eight owners and 250k miles? Full story for those who care are below the photos, but carnage first...

This was cyl. #1...seemed pretty bad:


but it kept getting worse until we got to cyl. #4 (which I'd suspected had been the cause of my long-term rod-knock):


My friend said they were the worst rod bearings he'd ever seen.

This story actually goes back quite a while: Mike W. may remember way back in 2008 at 5er Fest Reno commenting that my white 528e (at that time ~195k miles) had "light" rod knock. This worried me for a while, but it was "light" and I was doing regular (5k) oil changes with Castrol GTX 20w50, it was living an unstressed life tied to an automatic transmission, and the car quite frankly had other more pressing issues, so I slowly learned to ignore it "until it got louder." Later that fall (at 205k), I did a top-end "e2i" conversion and drove it to ~244k before putting it in storage due to financial issues in 2011. The rod knock never got louder, in spite of frequently spinning it well-past the engine's original 4,850 rpm redline.

In 2015 I took it out of storage with 244k on the clock, got it's running issues sorted and drove it until the automatic transmission decided to bleed-out one day at 246k. This prompted a manual transmission and differential swap to finally take advantage of the extra power. Even then the rod knock never got worse. That is until the day I couldn't get my drain plug out for an oil change...and after several failed weekends I folded and took it to a "reputable local shop" before I broke or stripped something.

When I picked up my car from this "reputable" shop, I noticed on the invoice that I'd been charge for six quarts of Synthetic 5w30...not the non-synthetic 20w50 I'd specifically requested over the phone when I'd made my appointment. The shop's head mechanic reassured me that they use the synthetic 5w30 all the time on m20s (in e30s) without issue, even on high mileage engines. I made a stink and he offered to change it, but I said I don't want them touching my car again and I'd change it at home, so he gave me a quart extra in case it leaked through my old seals until my next change and I took my car and left.

That said...I didn't actually end up changing the oil until ~3000 miles later, figuring that it wouldn't really hurt the once and they'd assured me they'd done it all the time without issue....and they were supposedly a "good shop with an excellent reputation". Well, about 2500 miles into the 5w30 the rod knock decided to make itself re-known, and by 3000 it was an absolutely horrid-sounding death-rattle. Changing the oil made no difference at thus point, it was over, the bearings were done. After an annoying few weeks of bad weather and delayed shipping, a friend came over (derrith on this forum) and provided some much-needed expertise, tooling and muscle to help swap all the rod bearings, and the photos above are the result.

The rod knock is completely gone, the crank itself looked undamaged, and the bottom-end of engine and oil pain were surprisingly clean for a 250k mile engine, no sludge, just a few coppery flakes from the above bearings (obviously). I do believe that the frequent changes with high-quality oil were the only thing that allowed me to continue driving it like that without it getting worse for another 60k+ miles before it finally went. In fact I don't think it was as loud in 2016 before the 5w30 as it was in 2008 before I started the 5k regimen of 20w50 Castrol GTX. I was glad to be right in suspecting all along (as early as 2009) that cylinder 4 was the culprit (at least the worst). The car had seven owners before me and was clearly never cared for...the head had been replaced at some point in the past due to the car clearly overheating (paint was dull-ish over the head on the original hood, indicating heat), and so it's no surprise it had rod bearing damage when I got it at 185k in 2007. Either way, I'm glad that I was able to keep the original engine (well bottom-end) going even longer. I'm hoping to fully rebuild it at 300k to specifications which make more sense for the top-end modifications I've done, so I think another 45k isn't a tough ask considering the bearings are shiny and new. I'm now trying Kendall oil for the higher ZDDP content and claim to quality, so we'll see how hard an ask that actually is...

[tig]

Correlation != causation, but synth oil seems as likely the cause as anything else, given the age.

Super interesting nonetheless. Thanks for sharing!

[tn535i]

Yep those are pretty trashed.

When these cars were engineered there was very little 5w-30 in use and engine intended to use thicker oils than that. As ANY engine gets a lot of miles piled up well north of 100k and has certainly some internal wear it's not a bad idea to consider sticking with or going to heavier oil to maintain good pressure and lubrication. So I think the mechanic insisting 5w-30 was OK in it accelerated the demise of an engine already with some question marks.

I have personal experience near opposite... we bought an e30 (intended for a parts car) with an M20 that had both overhead and slight rod noises and a low oil press light at idle with almost 300k. The stickers on the windshield showed 5w-30 oil changes but as soon as the oil was changed to 20w-50 it quieted down and then we decided to run it, which it did, for a couple years before breaking a rocker arm. Fixed that and it did maybe another 40-50k miles with, to the best of my knowledge, original bottom end. I'm convinced the cam wear began with the neglect of regular oil changes and then only modern too thin oil out of the big ole drum for $29 at the lube shop used in every other car.

Not trying to re-ignite an oil debate but if you have an old BMW read the owner's manual and put the proper oil in it, preferably one with higher ZDDP.

[ElGuappo]

Logan showed me the pics when you guys did the work, I was impressed.
5w30 has no business near any 80s BMW motor, whoever said that was a crackhead.

[davintosh]

That's pretty impressive. I'm always surprised at what these engines can have wrong with them and still run.

5w30 has no business near any 80s BMW motor, whoever said that was a crackhead.

What he said. I remember the way my old M20 rattled and flashed the oil light when it was running 10W30 during a freakishly warm February one year. Can only imagine what running 5W30 would do in warmer climates.

[Mike W.]

The shop putting in 5-30 is just plain stupid, or more likely lazy since they probably had it in a drum they pumped it out of. Granted SF isn't exactly Death Valley, it's not Alaska either, so going 3 grades thinner on a high mileage engine, even giving them the benefit of the doubt in likely not knowing the history means they just didn't want to have to do anything outside the normal routine.

A bit of a however though. Especially in the second picture, those bearing look odd. The babbitt isn't so much worn thru as missing. Usually bearings get worn, you can see lots of copper and some scoring, but they're still semi smooth, or at least not like those with much of the babbitting just plain missing and chipped away. That's odd. But at least Derrick didn't ignore things and end up with a spun bearing let alone a rod thru the block. Also says something about 80s BMWs when the "soft" ie cast iron not forged steel crank in the eta held up ok under more than severe conditions.

I remember back before e to i conversions were common and the popular wisdom was you had to use the td crank (forged steel) in an eta if you wanted to rev it or make any power out of it.

[duggi]

I knew the 5w30 wasn't going to do the engine any good, so wasn't surprised when the knock suddenly started getting louder, but was nonetheless disappointed in a BMW shop who should've known better. I'm the same, in that I'm surprised at how far one of these engines could go in such condition. While the 5w30 really did them in, there was obviously some long-term damage already happening to the bearings..all that damage doesn't happen in just a few thousand miles. And honestly, I always thought the stock b27 crankshaft would be more than strong enough to handle the added power/RPM, and this one at least has proven to be so. I will be going with the forged crankshaft when I rebuild, just because I can, but I don't actually think it's necessary after my experience so far. It will be interesting to see what the crankshaft really looks like when it comes out of the block in a few years though.

[Nebraska_e28]

Good info! Thanks for sharing.

[catso]

Just curious, did you mike the crank or plastiguage the new bearings to check your current clearance? Or did you have the crank ground? When a bearing gets eaten up like that I would suspect at least some minor crank wear too. One other thing; I remember doing an old Dodge truck crank once and the machinist who supplied me a nicely machined forged crank gave me a very small but highly efficient wirebrush for cleaning the oil supply holes very thoroughly, something I realized was an absolute necessity.

[duggi]

Just curious, did you mike the crank or plastiguage the new bearings to check your current clearance? Or did you have the crank ground? When a bearing gets eaten up like that I would suspect at least some minor crank wear too. One other thing; I remember doing an old Dodge truck crank once and the machinist who supplied me a nicely machined forged crank gave me a very small but highly efficient wirebrush for cleaning the oil supply holes very thoroughly, something I realized was an absolute necessity.

I bought the plastigauge kit, but didn't end up using it. the crank seemed fine, and I'm only hoping to get another 50k out of the engine before it comes out (at ~300k) for a full rebuild, including a forged 2.7 crank, so

[wkohler]

Wow seeing those bearings and hearing the crank is fine is amazing, especially given you already had a knocking sound, so it wasn't really smooth rotation as much as collision every time the crank interacted with whichever rod was making the noise. Like Mike, I really don't understand why the babbitt flaked away. That's nuts. Glad it's sorted and fingers crossed for many more miles of trouble-free operation.