[Skiy]

My 2012 135i dct N55 developed a rod knock so i took out the engine, disassembled everything and the rod bearing shells were one on top of the other and deformed. Upon closer inspection i noticed what look like fractures on the crank. This is in cylinder #6. The other bearings don't look worn but gouged a bit from the loose metal. Is this a common issues for the crank to crack and spin the bearings? i had a 2012 n54 z4 35is years ago that also spun the 6th rod bearing and the crank ended up having a crack as well which is supposedly forged. My 135i n55 crank should be cast but same issue. Could it be the dct thats harsh on crank and causes it to crack from twisting forces? Anyone have similar experience and a theory on whats causing this?

Current plan is to clean everything up and install the N55 m2 forged crank along with M2 oil pump, suction pump, oil pan and King bearings.

111k miles, no idea about history other then it has a hard life based on the crap condition and 3 -4 layers of paint underneath the wrap lol it all is getting stripped and will have 1m front conversion and repainted back to alpine white. I got it with engine misfiring, all the gaskets leaking and coils had some oil on the bottom of them. its a project car in the most literal sense

[Skiy]

trying to capture the crack which appears to extend beyond the bed of the journal into the little cranny where it meets the vertical part of crank

[whyzee125]

Damn that sucks. I see tons of N55 spun bearings but I've never seen a cracked crank before, that's really strange. Time for an LS engine?

[Skiy]

Originally Posted by whyzee125

Damn that sucks. I see tons of N55 spun bearings but I've never seen a cracked crank before, that's really strange. Time for an LS engine?

Haha thats a whole other can of worms to open up.
Unless it cracked from overheating but then why would it if bearings arnt worn.

[wheela]

Wow, that stinks, sorry to hear/see this. I don't think I heard of cracked crank before, but I think there have been a very small handfull of broken n55 cranks - certainly not common. As far as bearing spinning, I think the current thinking is it boils down to two main buckets: 1) not priming oil properly after replacing the oil filter housing gasket, 2) oil starvation during long, high-g left hand cornering (tracking the car). I like your plan for correcting this though!!

Edit: forgot to mention the long recommended oil change interval is also suspect by a lot of people as a contributing factor.

[wheela]

I'm also suspect of the DCT causing the crank to change speed so fast with its super fast shifting. Not saying that caused the cracks, but every now and then you hear about spun crank hubs. Anecdotally, it seems like they tend to be on DCT, or manual transmission cars. I always wondered if it was the crank changing rotational speed so fast that loosens up the hub.

[Skiy]

Originally Posted by wheela

Wow, that stinks, sorry to hear/see this. I don't think I heard of cracked crank before, but I think there have been a very small handfull of broken n55 cranks - certainly not common. As far as bearing spinning, I think the current thinking is it boils down to two main buckets: 1) not priming oil properly after replacing the oil filter housing gasket, 2) oil starvation during long, high-g left hand cornering (tracking the car). I like your plan for correcting this though!!

Edit: forgot to mention the long recommended oil change interval is also suspect by a lot of people as a contributing factor.

OFHG was not changed. All gaskets look original. If there was oil starvation i would assume other bearings would show wear. This car was on crappy tires so i doubt a lot of G forces could have been pulled. Plus it would just wear the bearings first. I think crack happened first. That would cause the crank to expand and bit into the bearing and spin it. Bearings were on on top of the other. That would block off oil supply and cause it to smash up bearing as it did. Regarding crank hub bolt i was not able to break mine loose. I used a 6 foot steel tube as leverage and only broke my pry bar in half that i stuck between crank and block at the back. Was not able to get it loose. Lol no way it can spin from just spinning the cams. Even with higher power it doesnt matter. Crank hub is in the front and sprocket just spins oil pump and timing chain which spins the cams. Cams dont really increase resistance with more power. If anything crank should snap in half. But again i have no idea of the history of this car. Supposedly it was some tuner shops car from Sacramento and in one post they mention meth. Maybe it happened under their ownership. I got the car very cheap knowing that it might be messed up but wasnt expecting rod knock. And the fact that i had this on an n54 and n55 is concerning as it might be a weak spot for these cars.

[Skiy]

Originally Posted by wheela

I'm also suspect of the DCT causing the crank to change speed so fast with its super fast shifting. Not saying that caused the cracks, but every now and then you hear about spun crank hubs. Anecdotally, it seems like they tend to be on DCT, or manual transmission cars. I always wondered if it was the crank changing rotational speed so fast that loosens up the hub.

Rear my previous post about trying to break free the crank hub bolt. 6 foot extension pipe couldnt loosen it. I broke a square pry bar in half that was holding crank in place and bolt didnt budge.

[carguy138]

Sorry to hear this Skiy. I had a bearing let go a few months ago. Ended up going with a built longblock with M2 N55 forged crank, VAC coated bearings and ARP hardware. It's sitting in the garage and I'll be doing the swap in the next couple months.

I have a feeling that there isn't simply any one cause of this and BMW just did a half assed job designing the motor. Could be insufficient R&D, cost cutting via material selection or something else. There was a thread on M3post alluding to the problem with the S65 rod bearings being linked to the torque to yield rod bolts. The theory was that over time the BMW OE speced rod bolts were losing tension or were improperly torqued from the factory. Supposed fix was VAC bearings and ARP hardware (the lower torque version so the rod ends don't get distorted).

I've been quite the BMW nerd for the past 20 years and it seems like every model has some obnoxious fatal flaw, fragile cooling systems and oil leaks galore. Whether it's timing guides on the N20, S54/S65/S85 rod bearings, oil leaks on everything, then all the stupid plastic found everywhere. To this day why can't they build a V8 that lasts more than 60-80k miles?

At this point when BMW launches a new model my first thought now is, "What's going to be wrong with this one".

I'll also add that I refused to believe this was a problem until it happened to me. I attributed the issue to people beating on cold motors, extended OCIs, serpentine belt getting sucked in through main seal and anything else you can think of.

I bought my 335i in 2014 with 20k miles (2nd owner); did all of the proactive maintenance, 4-5k OCI, ALWAYS warmed it up and let it cool down before getting on it, garaged it in the winter. No methanol injection or E85, mild tune, always ran Shell 93 gas and generally treated it very well.

Then on it's 3rd track day at 72k miles, it let go. I don't have a very positive view of BMW right now.

[wheela]

Originally Posted by Skiy

OFHG was not changed. All gaskets look original. If there was oil starvation i would assume other bearings would show wear. This car was on crappy tires so i doubt a lot of G forces could have been pulled. Plus it would just wear the bearings first. I think crack happened first. That would cause the crank to expand and bit into the bearing and spin it. Bearings were on on top of the other. That would block off oil supply and cause it to smash up bearing as it did. Regarding crank hub bolt i was not able to break mine loose. I used a 6 foot steel tube as leverage and only broke my pry bar in half that i stuck between crank and block at the back. Was not able to get it loose. Lol no way it can spin from just spinning the cams. Even with higher power it doesnt matter. Crank hub is in the front and sprocket just spins oil pump and timing chain which spins the cams. Cams dont really increase resistance with more power. If anything crank should snap in half. But again i have no idea of the history of this car. Supposedly it was some tuner shops car from Sacramento and in one post they mention meth. Maybe it happened under their ownership. I got the car very cheap knowing that it might be messed up but wasnt expecting rod knock. And the fact that i had this on an n54 and n55 is concerning as it might be a weak spot for these cars.

I'm glad your crank hub was nice and tight. But spinning crank hubs on n55 is a thing, just google n55 spun crank hub. I don't think anybody has conclusively proven the cause. But imagine an engine reved up to high rpm, and you drop the clutch quick (or a super fast dct shift) and slam the engine's rotational inertia again the mass of the car through the transmission and tires trying to gain purchase against the road. If your tires don't light up, then your engine rpm will pretty much instanaeously drop, and your crank hub will want to keep rotating. It's an impusle load, you obviously put a huge torque on it with a 6 foot bar, but it's not an impulse load. I wouldn't want to be holding onto a 12 foot bar hooking up to that kind of loading.

[Skiy]

Originally Posted by wheela

I'm glad your crank hub was nice and tight. But spinning crank hubs on n55 is a thing, just google n55 spun crank hub. I don't think anybody has conclusively proven the cause. But imagine an engine reved up to high rpm, and you drop the clutch quick (or a super fast dct shift) and slam the engine's rotational inertia again the mass of the car through the transmission and tires trying to gain purchase against the road. If your tires don't light up, then your engine rpm will pretty much instanaeously drop, and your crank hub will want to keep rotating. It's an impusle load, you obviously put a huge torque on it with a 6 foot bar, but it's not an impulse load. I wouldn't want to be holding onto a 12 foot bar hooking up to that kind of loading.

You do have a point. A sudden change in engine speeds can put a lot of force on it. The s55 has a larger front vibration damper which also drives the mechanical eater pump and an oil pump that pushes 18% more oil and has a secondary oil pump it rotated. So the s55 while using the same crank hub (different bolt though, which maybe is longer and threads deeper into the crank? Assumption) has a higher load being applied to the hub and it revs a bit higher so maybe thats why we see more of s55 spinning vs n55.

But can someone explain how a tuned engine running more power puts any additional force on the hub? Hub is in front. If anything more force when tuned will be put on rear of crank where it attaches to flywheel which puts that torque to the wheels. The front hub drives the oil pump and timing cams (and water pump on s55). Unless you add a higher rev limiter how will more power be the culprit for it having a higher change of spinning? There isnt any additional resistance added to it.

[wheela]

Originally Posted by Skiy

You do have a point. A sudden change in engine speeds can put a lot of force on it. The s55 has a larger front vibration damper which also drives the mechanical eater pump and an oil pump that pushes 18% more oil and has a secondary oil pump it rotated. So the s55 while using the same crank hub (different bolt though, which maybe is longer and threads deeper into the crank? Assumption) has a higher load being applied to the hub and it revs a bit higher so maybe thats why we see more of s55 spinning vs n55.

But can someone explain how a tuned engine running more power puts any additional force on the hub? Hub is in front. If anything more force when tuned will be put on rear of crank where it attaches to flywheel which puts that torque to the wheels. The front hub drives the oil pump and timing cams (and water pump on s55). Unless you add a higher rev limiter how will more power be the culprit for it having a higher change of spinning? There isnt any additional resistance added to it.

That's a good observation about s55 vs. n55, I bet that would account for this being a bigger issue with s55.

I have the same question about power level and spinning the hub, I can't see how power level is a factor? The only thing I can see is higher cylinder pressure = more force needed to push open the exhaust valves? Personally, I think high power is a bit of a red herring, and suspect it's not really power level, but more just beating on it in general. It's not a super common thing to have happen. I'm not sure which way the engine rotates vs. thread direction on the hub, but others have said it rotates in the direction where a quick deceleration would cause momentum of the hub and valve train etc. to try and unscrew the hub. If that's true, then it seems higher power could only help???

[Skiy]

i believe both are clockwise if i am not mixing things up. exhaust valves pressure? hm maybe, but it cant be much more.

[feuer]

From the bearing material that piled up on the crank the friction was so great that almost locked up, instead crank cracked, because the rest of the pistons still worked ok and the assembly wanted to rotate. This is probably only from oil starvation. High G aren't a must per say to starve these engines from oil. Frequent pulls on the straight in hight RPM's would have similar effect as the oil won't go to back down to the oil pan quick enough. Followed by hard braking makes this even worse as oil pool into the front of oil pan away from suction pipe. Cyl 6 is the furthers away from the pump so gets the oil last. Usually is either cyl 6 or cyl 5 which is the hottest. DCT isn't the reason. MT are more stressful for the crank.

[Skiy]

Originally Posted by feuer

From the bearing material that piled up on the crank the friction was so great that almost locked up, instead crank cracked, because the rest of the pistons still worked ok and the assembly wanted to rotate. This is probably only from oil starvation. High G aren't a must per say to starve these engines from oil. Frequent pulls on the straight in hight RPM's would have similar effect as the oil won't go to back down to the oil pan quick enough. Followed by hard braking makes this even worse as oil pool into the front of oil pan away from suction pipe. Cyl 6 is the furthers away from the pump so gets the oil last. Usually is either cyl 6 or cyl 5 which is the hottest. DCT isn't the reason. MT are more stressful for the crank.

Then why dont any of the other bearings show any wear? Including cyl 5, its second to last to get oil so it should have some wear but it doesnt. Thats whats weird about it.

[feuer]

Originally Posted by Skiy

Then why dont any of the other bearings show any wear? Including cyl 5, its second to last to get oil so it should have some wear but it doesnt. Thats whats weird about it.

The other bearing you hold on your hand to me with all honesty appears to be recently replaced. So it possible that all of them were replaced because of an previous issue with knock.

[Skiy]

Originally Posted by feuer

The other bearing you hold on your hand to me with all honesty appears to be recently replaced. So it possible that all of them were replaced because of an previous issue with knock.

It doesnt looks like engine has ever been opened up. Gasket was leaking and was aged. I will need to check manufacturing dates on the bearings. Do they not look like they have 111k miles?

[feuer]

Originally Posted by Skiy

It doesnt looks like engine has ever been opened up. Gasket was leaking and was aged. I will need to check manufacturing dates on the bearings. Do they not look like they have 111k miles?

The one doesn’t look like it no. Doesn’t show burn marks or anything.

[bbnks2]

Originally Posted by Skiy

trying to capture the crack which appears to extend beyond the bed of the journal into the little cranny where it meets the vertical part of crank

The crank "cracks" from the heat stress of the spun bearing (being starved of oil).

The spun bearing caused the damage to the crank. Not the other way around. Why did the bearing spin? It's just as common in both n54 and n55 engines (this forum is predominantly ~2011/2012 N55 owners so you see it a lot more here). Not likely to have anything to do with the crank.

My best guess is that it's the rods. High load at low rpm over time stretches the rod caps. Both n54 and n55 rods are virtually identical powder metal rods (sinter forged). Cheaper to produce for the almost all oems who use them now but also much stronger than a regular cast piece. Doesn't mean they will last forever.

Combined with oil starvation issues due to the design of the wet oil sump. There are 0 provisions in the oil pan to prevent starvation unless you buy an "M" car. Those engines spin bearings too. Probably most likely just a matter of "you have to pay to play."

[bbnks2]

Originally Posted by whyzee125

Damn that sucks. I see tons of N55 spun bearings but I've never seen a cracked crank before, that's really strange. Time for an LS engine?

every single n54 and n55 crank I have seen with a spun bearing has has heat stress cracks. If the cracks are superficial the crank can be cut but that reduces the thickness of the metal that has received nitride treatment (heat treatment to harden the journal surface). The resulting machined crank is weaker. Cast n55 crank is header to machine on and usually just garbage once trashed. Cast cranks needs more care in machining methods if they are bent etc. Cast is more brittle.

[bbnks2]

Originally Posted by Skiy

Rear my previous post about trying to break free the crank hub bolt. 6 foot extension pipe couldnt loosen it. I broke a square pry bar in half that was holding crank in place and bolt didnt budge.

yeah you need a 3/4 drive breaker bar and about 8ft+ of pipe lol. same shit to re-torque one. You need to "stretch" that bolt 270 degrees irc (goign off memory) after setting the initial torque.

Just another thought... Cylinder 5 on these engines have given people headaches for years. It seems that's the first cylinder to experience "knock" and constant timing pulls. It's probably the hottest running cylinder or something with airflow to/from that cylinder. You can find posts from tuners going back to 2007 trying to address cyl 5 timing via tuning. It is plausible you had timing issue son cyl 5 causing damage in the long run from constant knock/detonation on that cylinder. When a single bearing fails it does seem to very frequently be cyl 4 or 5. Could even be a harmonic thing. All of this is just anecdotal observation though.

[Skiy]

Originally Posted by bbnks2

yeah you need a 3/4 drive breaker bar and about 8ft+ of pipe lol. same shit to re-torque one. You need to "stretch" that bolt 270 degrees irc (goign off memory) after setting the initial torque.

Just another thought... Cylinder 5 on these engines have given people headaches for years. It seems that's the first cylinder to experience "knock" and constant timing pulls. It's probably the hottest running cylinder or something with airflow to/from that cylinder. You can find posts from tuners going back to 2007 trying to address cyl 5 timing via tuning. It is plausible you had timing issue son cyl 5 causing damage in the long run from constant knock/detonation on that cylinder. When a single bearing fails it does seem to very frequently be cyl 4 or 5. Could even be a harmonic thing. All of this is just anecdotal observation though.

When tightening the new hub bolt. How do you keep crank from spinning? Just that pin that you insert into flywheel when doing timing? I mean 8' pole vs small metal insert doesn't sound like it can end well