There I was lying on my back looking at this bronze colored crankhub, hoping that it was my eyes playing tricks on me. As the fluid from the uncovered pickup tube dripped onto my hoodie, I looked at my phone, which said that if I don’t change this component, my car will explode and I’ll be left picking up the pieces of my S55. Since I can’t trust you all for shit (the previous owner’s paperwork, the forums, all of you), I decided to do a deep dive on the crankhub anomaly for the S55, and for those that don’t know what that is, strap in, it’s about to get weird and German-ly complicated.
WTF is a Crank Hub
I’d go out on a limb and say that most cars have a harmonic balancer that’s bolted directly to the crankshaft. The timing chain or belt typically sits on a sprocket behind the timing cover, and there is a keyway that keeps things synced. Meaning as long as that key way doesn’t break (it’s rare) and you’ve set timing properly, things will stay working longer than the car will survive, most likely. This is what most of our golden era Japanese cars do, and it’s something you don’t even think about.
Well, let’s insert BMW and the S55 into the chat. They have an assembly called a crank hub that holds the harmonic balancer, timing chain, and oil pump on the crankshaft. The assembly is held together by the clamping force of a single central crank bolt and high-friction sintered thrust washers. There is no mechanical lock to prevent the components from rotating independently of the crankshaft if that friction is overcome. For the oil pump, it’s not a huge deal, but for the timing chain sprocket to move independently of the crankshaft, well, that’s a big fucking deal.
There has been speculation that the S55 shares roughly 75% of its architecture with the N55. The N55 was in 1,3,4,5 series and even the X series SUVs from the ’11s-’18s in the non-M variants. Basically, saying there are a lot of them out there. From a manufacturer’s standpoint, it’s more efficient to automate a press-fit assembly like this than to slot or pin the crank, because it requires greater precision and more manpower.
Some thoughts allowing a small amount of slip under extreme shock loads can help protect more critical components, such as crankshaft journals and main bearings, from absorbing those destructive forces. If it’s blown up, it doesn’t matter when it’s all in the scrap yard.
Then Why Aren’t all S55s and N55s Blown Up, Devin?
As stated, the N55 shares the same crankhub design but doesn’t share the same failures. The S55 with bolt-ons can easily achieve 500ft-lbs of torque at the wheels. It’s believed that the harmonic balancer and the assembly itself experienced far more torsional harmonics. The high-revving S55 operates in a higher stress envelope than the N55S. With the ease of making more power, moments of detonation and shock loads from a tuned DCT can have a hammering effect on the crankshaft. The S55 also has a mechanical water pump vs the N55’s electric pump, and an extra fuel pump driven by the cams, which could aid in counter torque to the assembly. In short, it may not be one big thing but many small things that cause this.
The system itself was designed for OEM power ratings, and it’s backed by data showing that fewer than 1% of stock cars suffer this failure. Cars that are considered stage 1 (450-500whp) around 5-10% and stage 2 (over 550whp) 10-20% suffering from a crank hub failure. The consensus is that if you track a DCT, launch from a standstill (a dig), and use aggressive high-rpm downshifts, you should be worried.
Still take these numbers with a grain of salt, since they aren’t reported by BMW. It may have been repaired under warranty or under some form of NDA. To find these numbers, you have to browse the internet to come up with a rough comparison, so you’re welcome because I did it for you. The broad consensus is that under 10% of all S55s have suffered a crankhub failure. It’s difficult to consider the data as concrete. In fact, some think it’s a snake, and others think it’s not the system giving out, but the components not being replaced. Rubber in a harmonic balancer does wear out, and we often don’t think of them as wear items, but when you think about it, they should be
Why must it be fixed?
The obvious reason to be thinking about it is to avoid blowing up an engine. Timing between the crankshaft and the camshafts not being in sync is, at best, a tow-truck ride home. These engines on the used market are still fetching $8K+, and that just doesn’t sound like a good number for my bank account. Even if it’s statistically unlikely to happen, if I can bring that risk down by 10-20%, that would be great. Think about it this way: if you could lose 10-20% of your total mass off your car or yourself, that would be pretty impressive, right? The F8x market is aware of this as well, and most buyers are looking for an upgraded crank hub before purchasing, especially in my 100k+ miles depreciation bracket. The question is, though, with the amount of work involved, the cost of an upgraded crankhub, and the cost of the specialty BMW tools, you are looking at no less than 3k to do it yourself, and to have a shop do it, you are around the same price. Is a 10–20% risk reduction worth it? At $3K, that’s nearly half the cost of a replacement new-to-you S55. That’s the math you have to sit with. That’s a decision that makes you really ponder.
The Top Crankers
There are two main ways the aftermarket has skinned this cat. You can go with key /pinned or spline-locked. The designs do away with the three-piece crank hubs altogether in favor of mechanically locked two and one-piece designs. This is the general consensus on the go-to brands right now. One disclaimer, though, if you are going to a shop, whatever they specialize in, do that one. Likely the one they have had the best experiences with and the most experience installing. If you plan on doing some of these yourself, be sure you are confident in your skills, because messing up an install can cause engine failure, and now you are out of money to put into that new-to-you engine.
One-Piece Pinned Hubs
These integrate the hub, timing sprocket, and oil pump sprocket into a single machined billet component, which is then physically locked to the crankshaft with pins. These are considered the ultimate fix for builds north of 700hp for their simplicity, since they remove friction entirely and replace it with mechanical lock. These are typically the most expensive as well. Since it’s physically pinned into the crank, this does require drilling into a forged crankshaft, which is time-consuming, and you’d better have a lot of carbide drill bits. You will also be more space-constrained than the two-piece units on this list, so more parts have to come off the front of the car (most likely). The top two brands are Gintania and MMR Performance.
Best for: Big power builds over 700hp. Most expensive, most permanent, most bulletproof.
Two-Piece Pinned Hubs
These kits usually combine the hub and timing gear into a single piece, while the oil pump gear remains separate and is mechanically keyed to the hub assembly. The two separate halves make installation slightly easier by giving you more room to maneuver. Everything in the F8x engine bay is cramped and fitted tightly. Getting another inch or two is a game-changer (shut up, don’t even say it). These are pinned into the crank the same way as the one-piece designs, and they are by far the most popular. They won’t save you much vs the one-piece ones, but they are typically cheaper since it’s easier to manufacture the two halves. It’s still complex because it’s two halves that are keyed (one to the crank and one from the oil gear to the hub assembly). Top brands are SSR Performance, Vac Motorsports, Evolved Automotive, Keltho, and RK Autowerks.
One-Piece Unpinned Hubs
These replace the multi-piece factory hub with a single billet unit but do not use pins to lock the assembly to the crankshaft. These are considered the more DIY friendly and affordable options. However, they aren’t recommended for DCT cars or those at or above 600hp. It is eliminating the gears from independently spinning, but you still risk the entire assembly spinning. Often, these are paired with a crank capture, a guard that surrounds the crank bolt and is bolted to the crank hub assembly so the bolt can’t move independently of the crank hub assembly. MMR Performance is the only brand I found that makes these.
Best for: Budget-conscious manual trans cars under 600hp.
One-Piece Spline Lock Hubs
A unique middle-ground option is a spline lock, a one-piece design that uses hardened splines to “bite” into the crankshaft rather than pins. There are no holes drilled and it can be removed if necessary. Although if you do take it off, it will leave permanent spline tracks on the crankshaft. It still requires a specific expertise to ensure the splines engage correctly. As I mentioned earlier, if not, it will lead to failure.
Best for: People who want a mechanical lock without drilling the crank.
Even with all these options, the consensus is still to run a crank bolt capture as added insurance, so the crank bolt has almost no chance of vibrating loose.
Is this stock or what?
Don’t be like me and wait months after buying the car to confirm whether it was done. Mine has a crank bolt capture on it, so I thought, hmm, surely someone would do the whole system right? The easiest way to check is to grab a borescope and stick it down the timing chain from the oil fill cap. The stock crank hub will have a serial number on it and look like it’s not one piece. Almost all aftermarket crankhubs will be shiny, since most are billet and appear to be made of fewer than three pieces. If it’s a pinned unit, you may be able to see the heads of the locking pins in the oil gear. Some people also go by the inspection caps on the front of the engine. If they appear new or have marks on them from prying them off, that could be a sign, but forget that. Just borescope it. Outside of that, the obvious service history with receipts or articles like this one. That’s about all you can do.
Bonus: Maybe think about changing these too while you’re at it (harmonic balancer)
Again, this is often overlooked, but think about it, usually other secondary issues would go away if it were all based on shock alone. BMW’s have always had harmonic issues, just think about all the engines that needed rod bearings every 60k miles. It’s not just the high-revving, either, because a S2000 revs to 8500-9000 rpm, and that was never routine maintenance. Instead of a turbo, consider upgrading your harmonic balancer to an ATI unit or a new OEM unit. Give that crank the best life that you can, I mean, while you are in there, right?
Now what?
I weighed my options and I’m still undecided. So I’m putting it to you. Pinned or spline? One-piece or two? Go vote in the polls over at @imdevingiles and tell me what you’d trust. Because at this point, you’re building this car with me whether you like it or not. If you prefer Substack, we are on there now as well, here: @imdevingiles2