Piston Skirt Coatings for Alusil and Lokasil Engines: What Matters and Why

Modern aluminum engine blocks such as Alusil and Lokasil rely on exposed silicon crystals within the cylinder wall for wear resistance. Because these bores do not use iron liners, pistons require a ferrous skirt coating to prevent aluminum-on-aluminum contact and galling. Overlooking this detail can turn a healthy engine into a bore-scored core in a hurry, especially without proper maintenance.

Why Ferrous Coatings Are Essential

Without a ferrous barrier on the piston skirt, the aluminum piston can contact the silicon-rich aluminum bore surface. Once that happens, the result is rapid galling and, soon after, bore scoring. Either outcome can force an expensive repair—boring and sleeving the block or replacing it entirely.

Two Proven Approaches: Ferrostan and Ferroprint

Mahle developed two widely used skirt coatings for Alusil/Lokasil applications—both seen in high-performance engines from Porsche, BMW, and Mercedes-Benz. They differ in construction, application method, and long-term durability.

Ferrostan (Electroplated Iron/Tin)

• What it is: A dual-layer electroplated coating—an iron layer for wear resistance topped with a thin tin layer to aid break-in.
• How it’s applied: Electroplating bonds the iron layer metallurgically to the piston skirt.
• Why it matters: The bonded iron layer is extremely hard and durable, offering excellent long-term protection under high load and temperature.
• Trade-offs: Electroplating involves chemicals and processes with environmental considerations; many manufacturers reduced or phased out this method in favor of alternatives.

Ferroprint (Resin with Stainless Particles)

• What it is: A polymer-resin coating infused with stainless steel particles.
• How it’s applied: Typically screen-printed or sprayed onto the skirt, then cured.
• Why it matters: Easier to apply and generally more environmentally friendly from a manufacturing standpoint.
• Trade-offs: While effective within its limits, it does not typically match Ferrostan’s long-term durability in severe service.

Durability and Failure Modes

Historical field experience shows Ferrostan holds up exceptionally well in demanding conditions, with very low incidence of bore scoring when used correctly. Ferroprint offers a cleaner production process and solid performance for many applications, but it is generally considered less durable over extended high-load, high-heat use.

If either coating is damaged during assembly—or simply worn through in service—the aluminum piston skirt can contact the Alusil/Lokasil bore. That metal-to-metal contact accelerates wear, leading first to scuffing and galling, then to visible scoring that compromises sealing and oil control.

Best Practices for Builders and Owners

• Specify the right pistons: Use pistons specifically engineered for Alusil/Lokasil with an appropriate ferrous skirt coating. As of writing this, Mahle Motorsport is the only aftermarket piston manufacturer that offers the required coatings.
• Match use to coating: Ferroprint is not compatible with other cylinder bore technologies, like Nikasil or even cast iron cylinder bores.
• Inspect before assembly: Verify continuous, intact skirt coverage; avoid nicks or handling damage that could become failure initiation sites.
• Use correct clearances: Follow the piston manufacturer’s clearance and finish requirements for Alusil/Lokasil bores.
• Control lubrication and break-in: Proper assembly lube, first-start procedure, and early oil changes reduce risk during the most vulnerable hours of operation.

Bottom Line

In Alusil and Lokasil engines, skirt coating choice is not cosmetic—it is fundamental to reliability. Ferrostan provides the most robust long-term protection, but it's not used anymore; Ferroprint is a viable, cleaner-production alternative when used within its operating limits, requiring the correct bore prep, clearances, and engine oils. Choose wisely, assemble carefully, and you greatly reduce the risk of bore scoring.

How to Assemble Piston and Cylinder Assemblies for Optimal Engine Performance

Once your pistons, rings, and cylinders are properly cleaned and prepped, it’s time to move on to assembly. In our latest YouTube video, we demonstrate the key steps to assemble piston and cylinder assemblies. Follow this guide for a thorough breakdown of the process to ensure your engine build is set up for success.

Pre-Assembly Preparation

Before starting, make sure all components are ready:

• Clean Components: Cylinder bores should be cleaned with denatured alcohol and Kimwipes to remove all residue.
• Ring Gaps: Piston ring gaps should be measured and adjusted as necessary.
• Wire Locks Installed: Install wire locks on one side of your pistons prior to assembly.

Proper piston ring orientation

Key Considerations for Piston Rings

1. Check for Directionality:

   • Many piston rings are directional. Look for markings such as “TOP” to ensure they are installed correctly.
   • Note: “TOP” indicates the orientation, not the position on the piston.

2. Match Rings to Grooves:

   • Ensure each ring is fitted to the correct groove. Rings of the same thickness may differ in function, so consult the manufacturer’s instructions.
   • Improper placement or orientation can lead to oil consumption, blow-by, and cylinder bore scuffing.

3. Radial Back Clearance:

• Verify that no part of the ring’s inside diameter protrudes beyond the piston ring land. Insufficient clearance can cause severe engine damage.

Piston ring radial back clearance

Installing Piston Rings

While many manufacturers recommend using ring pliers, many engine builders prefer the “walk” or “spiral” method:

1. Start with the oil control ring.
2. Gently spiral the ring onto the piston, taking care not to bend, distort, or break it.
3. Proceed with the second ring, then the top ring.

Staggering Ring Gaps

Once the rings are installed, stagger the end gaps to prevent alignment. Misaligned gaps can lead to blow-by and reduced compression.

Using a Ring Compressor

Modern thinner rings require extra care during installation. We recommend using a tapered sleeve ring compressor for better control and reduced risk of damage:

1. Lubricate the Components:

   • Apply lubricant to the piston skirts, rings, and the inside of the tapered sleeve compressor.

2. Load the Piston:

   • Push the piston into the tapered sleeve until the skirt protrudes slightly.

3. Prepare the Cylinder:

   • Lubricate the cylinder bore and position the ring compressor on top.

4. Insert the Piston:

   • Slide the piston through the tapered sleeve into the cylinder bore, applying gentle, even pressure.
   • Hold the tapered sleeve firmly to prevent gaps between the sleeve and the cylinder.

5. Adjust if Needed:

• If the piston stops or the ring catches, stop immediately. Reposition the sleeve and try again.
• Alternating pressure at the top and bottom of the piston crown can help guide the rings into the bore.

Installing the piston using a tapered sleeve ring compressor

Final Steps

With the piston fully seated in the cylinder, your assembly is ready to be installed onto the engine. Take your time and double-check your work to avoid costly mistakes.

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Final Thoughts

Proper assembly of pistons and cylinders is crucial for engine performance and longevity. By following these steps and using the right tools, you’ll minimize the risk of damage and ensure your engine operates smoothly.

For a full visual walkthrough, watch our video: How to Assemble Piston and Cylinder Assemblies. If you have any questions or need advice, leave a comment—we’re here to help.

Watch the Video Now!

Installing Piston and Cylinder Assemblies: A Step-by-Step Guide

With your short block complete and ARP head studs installed, the next step is assembling and installing your pistons and cylinders. This process is critical for ensuring proper deck height and a reliable engine build. Our latest YouTube video walks you through this procedure. Below is a detailed guide to help you achieve success.

Step 1: Check and Adjust Deck Height

Deck height—the distance from the piston crown to the top of the cylinder—is a crucial measurement. Follow these steps to check and adjust it:

1. Temporary Assembly:

   • Install one piston and cylinder assembly without sealant, base shims, or gaskets.
   • Push the piston through the cylinder to expose the wrist pin bore.
   • Optionally, pre-install the wrist pin into one piston boss for easier alignment.

2. Align with the Connecting Rod:

   • Slide the assembly onto the engine and line up the piston with the connecting rod.
   • Lubricate and insert the wrist pin through the rod, but do not install the wrist pin clip yet. This assembly will be removed later.

3. Measure Deck Height:

   • Rotate the engine to bring the piston to top dead center (TDC). Hand-feed the timing chains to prevent them from binding.
   • Measure the deck height using a specialized deck height tool or a dial/digital caliper.
   • Target Deck Height: 1 to 1.5 mm. If it is less than 1 mm, base shims will be required.

4. Select Base Shims:

• LN Engineering offers copper base shims in 0.25 mm, 0.5 mm, 0.75 mm, and 1 mm thicknesses. Choose the correct thickness to achieve the desired deck height.

Step 2: Prepare for Final Assembly

1. Disassemble and Clean:

   • Remove the piston and cylinder assembly.
   • Clean the cylinder base and case deck using denatured alcohol and Kimwipes to ensure a spotless surface.

2. Apply Sealant:

• Use a non-hardening sealant on the cylinder base. If using base shims or gaskets, apply sealant to both sides.
• Recommended Sealants:
• Elring Curil T (used in this guide)
• Curil K2
• Dow Corning 730 (top pick by most engine builders, but $$$)

Step 3: Final Assembly

1. Install the Assembly:

   • Slide the cleaned Porsche piston and cylinder assembly back onto the engine, aligning the wrist pin bore with the connecting rod.
   • Insert the wrist pin into place and install the second wrist pin clip.

2. Secure the Clips:

   • Before installing the second clip, cover all openings in the engine case to prevent accidental loss of the clip inside the engine.
   • Verify that both sides of the piston have wrist pin clips installed securely.

3. Seat the Cylinder:

• Push the cylinder firmly onto the engine case. Use a Porsche cylinder hold-down nut to secure the assembly until the heads are fitted.

Step 4: Repeat for Remaining Assemblies

Repeat the process for all remaining piston and cylinder assemblies before proceeding to install your Porsche cylinder heads.

Tips for Success

• Take Accurate Measurements: Proper deck height ensures optimal engine performance and prevents damage.
• Handle Clips Carefully: Wrist pin clips can easily eject during installation. Always work with caution and keep spares on hand.
• Use High-Quality Tools and Materials: Proper tools and sealants make the process smoother and more reliable.

For a detailed demonstration of this process, check out our video: Installing Piston and Cylinder Assemblies. If you have any questions or need further guidance, leave a comment—we’re here to help.

Watch the Video Now!