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For performance enthusiasts and BMW aficionados, the S55 engine is a masterpiece that delivers power, precision, and sheer driving pleasure. However, it's no secret that the factory crank hub design has raised concerns over its durability, making aftermarket crank hub kits a popular upgrade choice. In this article, we will explore and compare three popular options for upgrading the crank hub: the one-piece non-pinned and pinned kits from MMR Performance as well as the multi-piece pinned kit from Evolve Automotive. Let's delve into the pros and cons of each to help you make an informed decision.

The Problem with the OE S55 / N55 Crank Hub Design



The factory crank hub is a three-piece unit with high-friction thrust washers between the timing gear sprocket components, all held together by a single, high-torque crank bolt. The issue here is that from either a high shock loading event like an ultra fast DCT gearshift or from a loosened crank bolt (due to the engine’s vibrations and harmonics over time), the timing sprockets are able to slip rotationally from the hub and crankshaft itself. If that happens, the engine's timing is no longer set accurately and you run the extremely high probability that your pistons will come into contact with the intake or exhaust valves. At that point, depending on severity, you may be left with a very large and very heavy paperweight along with a bill for a new engine. At a minimum, you're looking at a full engine teardown to replace any pistons, valves, and other components that are damaged.

Before you panic, the chances of this happening on a stock engine are extremely low, but with every modification resulting in a bump in power, your risk goes up. Because the S55 can reach 600hp relatively easily, it is recommended that you address the OE crank hub if you're crossing the 500hp mark.

The Solutions



Thankfully we offer several options based on both your planned power levels as well as how involved of an install process you're willing to accommodate. Each available kit includes different features so we'll thoroughly review the benefits of each and any potential downsides.

MMR One-Piece Crank Hub with Capture Plate:



MMR's single-piece non-pinned crank hub with capture plate is a simple and straightforward upgrade option. It replaces the stock crank hub with a one-piece design, eliminating the factory thrust washers that may be prone to slipping and allowing the timing gears to rotate independently of the hub and crankshaft. Additionally, the included capture plate locks the crank bolt into place so there is no possibility that it can become loose over time. The non-pinned kit is often considered a budget-friendly option and is relatively easy to install because it does not involve drilling your crankshaft to create the keyways for a pinned-kit to lock into. However, it may not be the most robust solution for very high-performance applications.

Pros:

• Affordable: The one-piece non-pinned kit is generally the most cost-effective option.
• Easiest / Most Cost-Effective Installation: Because the crankshaft doesn't require drilling, it has the simplest the installation process making it more accessible to DIY enthusiasts.
• One-piece crank hub eliminates the OE thrust washers and eliminates timing gear slip relative to the hub.
• Includes capture plate to prevent the crank bolt from loosening.

Cons:

• Limited Performance: While this kit is a big improvement over the stock hub, it won't offer the same level of reliability as the pinned options in high-hp builds.
• Risk of Slippage: Without pinning, there's still a potential risk of the entire crank hub slipping relative to the crankshaft, especially under max engine stress; typically a violent DCT gearshift in high-hp engine builds.
• Not recommended for DCT-equipped vehicles outputting 600+hp.

MMR One-Piece Crank Hub - Pinned:



The one-piece pinned kit is a step up from the non-pinned variant and addresses its primary shortcoming. Like the non-pinned kit, the one-piece design eliminates timing gear slip relative to the hub. However the incorporated lock-pins improve the hub's connection to the crankshaft, addressing the risk of the entire crank hub slipping relative to the crankshaft itself. Pinned crank hub kits offer the ultimate in reliability and are highly recommended when approaching 700hp or more.

Pros:

• Enhanced Reliability: The addition of pins locks the crank hub to the crankshaft significantly improving the overall robustness of the crank hub and potentially eliminating independent rotation between the crank hub and crankshaft.
• One-piece crank hub eliminates the OE thrust washers and eliminates timing gear slip relative to the hub.
• Includes crankshaft drilling template/jig as well as a new replacement crank bolt.

Cons:

• Higher Cost: Compared to the non-pinned kit, the single-piece pinned version is pricier due to the inclusion of the pins and the drilling jig.
• More Complicated Install: Keyways must be drilled into the OE crankshaft, which may require professional installation.
• Does not include capture plate to prevent the crank bolt from possibly loosening over time, but it can be added separately for additional cost.

Evolve Two-Piece Pinned Crank Hub:



The multi-piece pinned kit from Evolve offers all the same improvements as MMR's pinned crank hub with one significant difference. The crank hub itself comes in 2 pieces, while still eliminating the factory thrust washers. Instead, one timing sprocket is part of the hub and the other is locked in place by alignment tabs. The multi-piece construction allows for a marginally easier install due to the space constrictions inherent with replacing the crank hub while the engine is still in the car. This sophisticated design utilizes multiple components and locking pins to ensure the utmost strength and reliability under the most demanding conditions.

Pros:

• Enhanced Reliability: The locking pins improve the overall robustness of the crank hub in high-hp engines and potentially eliminate independent rotation between the crank hub and crankshaft.
• While a multi-piece design, the OE thrust washers are still eliminated ensuring no timing gear slip relative to the hub.
• Includes crankshaft drilling template/jig.
• Offers marginally easier installation vs one-piece design under space constraints.

Cons:

• Premium Price: Due to the slightly more complex 2-piece design, Evolve's pinned crank hub kit comes in at the highest price of the 3 choices.
• More Complicated Install: Keyways must be drilled into the OE crankshaft, which may require professional installation.
• Does not include capture plate to prevent the crank bolt from possibly loosening over time.
• Does not include a new replacement crank bolt.

Conclusion:

Upgrading the crank hub on BMW's S55 engine is a crucial step for ensuring long-term reliability and peace of mind, especially for high-performance applications. Each aftermarket crank hub kit option has its merits, ranging from affordability to ultimate performance. If you're on a tight budget and seek a moderate improvement with the easiest install, the single-piece non-pinned kit could be a suitable choice.

However, if your goal is to push the limits of performance, the pinned kits stand out as the ultimate solutions, providing unrivaled durability and security. Ultimately, the best choice depends on your performance goals, budget, and potentially whether your vehicle is equipped with a manual transmission or DCT. Remember, when it comes to your BMW S55 engine, investing in a high-quality aftermarket crank hub kit is an investment in the heart of your driving experience.