Unless you rebuilding an engine with little to no budget you're going to upgrade to aluminum forged pistons. Besides being stronger than cast pistons, the lighter aluminum cuts parasitic power loss. That translates into more power at the wheels.
Recently Mahle showed us that there are distinct differences between their 4032 and 2618 alloys. The choice to use one over the other depends on your project. Are you building a street/strip machine or an all out racer? Choosing the wrong one can be devastating. To better understand, we'll let MAHLE explain.
4032 is a high-silicon, low-expansion alloy. Pistons made from this alloy can be installed with tighter piston to bore clearance, resulting in a tighter seal with less noise. 4032 is a more stable alloy, so it will retain characteristics such as ring groove integrity, for longer life cycle applications. Relative to 2618, 4032 is a less ductile alloy, making it less forgiving when used with boosted and/or nitrous applications.
The majority of Mahle forged PowerPak kits are made with 4032 alloy and require no additional piston-bore clearance. Mahle pistons are perfectly engineered to allow for the proper clearances assuming normal operation. For example, Mahle pistons for a small block engine will provide proper .0025"-.0030" clearances--right from the box.
2618 is a low-silicon, high-expansion alloy that is used for extreme-duty racing applications such as NASCAR, ALMS, etc. Due to its high-expansion characteristic, this alloy is engineered with additional piston to bore clearance. At the start of a cold engine, the pistons expanding process can be heard and is commonly referred to as the "piston slap". Once the engine warms up the noise subsides as the piston expands to its running clearance. 2618 is a more ductile alloy and grants higher tolerances with higher resistance to detonation. The forgiving characteristics allow for the most extreme conditions, but longevity is eventually negotiated after countless heat cycles.
Mahle pistons are designed for specific applications with the alloy that is best suited for that particular application.