Aluminum blocks are light, but aluminum makes a terrible bearing surface for piston rings — it galls, scuffs, and wears into an oval within thousands of miles. For decades, the fix was pressing cast-iron sleeves into the aluminum block. That works, but it's heavy, it limits how thin the cylinder walls can be, and the dissimilar metals expand at different rates (iron ~11 µm/m·°C, aluminum ~23 µm/m·°C), creating gap and distortion issues at temperature.

Modern engines increasingly skip the liner entirely and spray a microscopically thin steel coating directly onto the bare aluminum bore. The process is called plasma transferred wire arc (PTWA) or rotating single wire (RSW). A wire of low-carbon steel is fed into a plasma torch (~15,000°C) inside the cylinder. The plasma vaporizes the wire and atomized particles fly outward at supersonic speed, splatting onto the roughened aluminum wall and building up a coating roughly 0.15–0.3 mm thick.

What you get:

• Iron-oxide-rich surface with embedded porosity (4–8% by volume) that traps oil — better lubrication than a smooth honed iron liner
• Weight savings: roughly 1.5–2 kg per V8 block versus iron liners
• Better heat transfer: thin coating bonded directly to aluminum conducts heat to the water jacket far better than a press-fit liner with its insulating air gap
• Thinner cylinder walls mean closer bore spacing and more displacement in the same block footprint

Real-world example: The Nissan GT-R's VR38DETT uses PTWA-coated bores. So does the Ford Shelby GT350's Voodoo 5.2L flat-plane V8, the C8 Corvette's LT2, and most modern Mercedes-AMG engines. Bugatti's W16 uses it too. The Nikasil coating used by BMW in the 90s was an earlier electroplated nickel-silicon-carbide version — it worked great until high-sulfur US fuel ate the coating and BMW had to warranty entire engine blocks.

Rule of thumb for honing: A coated bore needs a finer final hone than iron — typically Ra 0.3–0.5 µm versus 0.8–1.2 µm for iron. The coating's inherent porosity provides oil retention, so you don't need the deep crosshatch valleys iron requires. Honing too aggressively strips the coating, exposes aluminum, and the bore fails catastrophically within hours.

The downside: you cannot bore oversize and rebuild. Once the coating is worn or damaged, the block goes back to a specialized facility for re-coating (around $400–$800 per cylinder), or it's scrap. The aftermarket rebuild culture that thrived on iron sleeves doesn't really exist for these blocks yet.