You Can’t Bore the BMW S55 Engine Block: A Deep Dive into Arc-Sprayed Iron Liners

The BMW S55 engine—powering the M3, M4, and M2 Competition—is a masterpiece of engineering that perfectly balances power, durability, and weight. If you’ve ever considered boring out the engine block, either due to damage or as part of a high-performance build, you’ll run into a unique challenge: the S55’s cylinder walls aren’t like those of older engines. Instead of traditional iron liners, BMW uses a spray-on iron coating, also known as an arc-sprayed iron liner, to reinforce the block.

Let’s explore why the S55's block design makes boring impossible, and how this innovative process contributes to the engine’s outstanding performance:

What is Arc-Sprayed Iron Coating?

The BMW S55’s engine block is made from aluminum for weight savings, but the aluminum cylinder walls alone wouldn’t be durable enough to handle the demands of a high-output engine. BMW’s solution? They apply a thin layer of molten iron particles directly onto the aluminum cylinder walls.

In this process, iron particles are sprayed at high velocity onto the surface, forming a uniform layer that’s typically 0.15 to 0.3 millimeters thick. This coating creates the wear-resistant surface needed for the engine’s high-performance demands—without the bulk of traditional iron liners.

Why BMW Uses Spray-On Iron Liners

This design is more than just clever engineering; it’s a deliberate choice to optimize multiple aspects of the S55 engine. Here’s why BMW chose this approach over conventional liners:

1. Durability Without Bulk

The iron coating provides excellent wear resistance, reducing the friction between the piston rings and cylinder walls. This protection is essential given the high pressures and temperatures the S55 endures—especially in modified or turbocharged applications.

2. Better Heat Dissipation for Performance Engines

Aluminum blocks offer superior heat dissipation compared to iron, helping the engine stay cooler under high loads. By applying only a thin iron coating, the engine retains the thermal efficiency of aluminum while still benefiting from iron’s strength. This is critical for maintaining performance during extended use or spirited driving.

3. Lightweight Design

Traditional cast-iron liners would have added significant weight to the block, impacting the car’s handling and acceleration. The arc-sprayed iron layer adds minimal weight, keeping the engine light while ensuring it can handle high loads and power. For a performance car, every ounce saved contributes to better agility and balance.

4. Improved Sealing for High Boost Applications

The iron layer also helps with cylinder sealing, which is crucial for engines running high boost pressures. With tighter sealing, the S55 can reliably push higher power levels, making it a favorite among tuners and builders.

Why You Can’t Bore the BMW S55 Block

Boring a block involves removing material from the cylinder walls to enlarge the bore size, usually to increase displacement or accommodate oversized pistons. However, with the S55’s spray-on iron coating, boring isn't an option for several reasons:

1. The Iron Layer is Extremely Thin
   At just 0.15 to 0.3 millimeters, the iron coating is too thin to allow for boring. If you attempt to machine the cylinder walls, you’ll strip away this crucial layer, exposing the underlying aluminum.

2. Aluminum Alone Can’t Handle the Wear
   Once the iron layer is removed, the exposed aluminum won’t provide the necessary wear resistance for the piston rings. Without the iron coating, the engine’s lifespan would be severely compromised.

3. Precision Matters—You Can’t Reapply the Coating Outside the Factory
   The spray-on iron coating is applied with specialized equipment during the original manufacturing process. Reapplying this coating in an aftermarket setting isn’t feasible—it requires precise factory-controlled conditions to bond properly to the aluminum block.

4. Over-Boring Risks Heat Dissipation Issues
   Even if re-coating were possible, increasing the bore size would reduce the block’s ability to dissipate heat effectively. The S55’s design relies on carefully calculated heat management to maintain performance, and altering the bore size could disrupt this balance.

What are Your Options for More Power?

While boring the S55 block isn’t an option, there are still ways to enhance its performance. One of the most effective alternatives is sleeving the block.

Sleeving the S55 Engine Block

Cylinder sleeves are an excellent way to overcome the limitations of the spray-on iron coating. This process involves machining out the aluminum cylinder walls and installing iron sleeves inside the block. These sleeves are durable enough to withstand high boost pressures and allow for the use of larger pistons, giving you more flexibility for power upgrades.

Benefits of Sleeving:

• Increased Displacement: Sleeving allows you to fit oversized pistons for higher displacement.
• Improved Durability: Iron sleeves are more durable and can handle extreme pressures and temperatures better than the original arc-sprayed coating.
• Rebuild-Friendly: Sleeved blocks can be re-bored in the future if needed, unlike the factory arc-sprayed coating.

Challenges of Sleeving:

• Sleeving the block is a complex, precision task that requires the expertise of a specialized machine shop.
• Cost: Sleeving is more expensive than traditional boring due to the labor and precision involved.
• Weight Gain: Although minor, the addition of iron sleeves does increase the overall weight of the engine.

Other Performance Solutions Without Sleeving or Boring

If you prefer to keep the stock block without sleeving, there are still several ways to get the most out of the S55:

• Forged Pistons and Rods: Upgrading the internals with forged components increases the engine’s durability and allows for more boost and higher RPMs.
• Turbo Upgrades: Swapping out the factory turbos for larger or hybrid turbos is a common way to increase horsepower.
• ECU Tuning: Advanced tuning with a standalone ECU lets you safely push the engine to higher power levels by optimizing fuel, air, and ignition parameters.
• Performance Camshafts: Upgrading to high-lift camshafts can improve airflow and boost power without changing the block.

Conclusion: A Masterclass in Modern Engine Design

The BMW S55’s arc-sprayed iron liners are a perfect example of how modern engines achieve high performance, reliability, and lightweight design. This innovative approach provides the durability of iron, the heat management of aluminum, and the weight savings that enhance vehicle dynamics. However, the trade-off is that boring the block isn’t an option—but with so many other tuning opportunities, there’s no shortage of ways to push the limits of this engine.

Whether you’re planning a turbo upgrade or exploring performance camshafts, understanding the nuances of the S55’s design will help you make informed decisions. This block is engineered to handle serious power, but it demands respect for its unique construction. Stick within its design parameters, and you’ll have an engine that can take you to the next level.

BMW S55 Applications:

2 Series

2018-2020     F87     M2 Competition

3 Series

2012-2018     F80     M3

4 Series

2014-2020     F82     M4
2014-2020     F83     M4