most people just say to make sure it's a 2618 alloy with the proper ring spacing and gap...

All ROSS Racing Pistons (since 1985) have been forged from 2618 billet bar stock and have been heat treated and aged to a T61 condition (2618-T61). 2618-T61 is the material that is used when fatigue resistance and durability are of prime importance by all premium racing piston manufacturers. This material has less than 1% silicon particulate content. High silicon content pistons (forged from 4032 or MS75) will not stand up to the most extreme stress placed on many racing pistons. If a crack starts to form in a high silicon piston it will continue until the piston experiences a catastrophic failure. Simply stated, adding sand to the aluminum only makes it more brittle. In the rare instance of a 2618-T61 piston cracking, the crack will continue to an area where the stress is not as great and will then stop. Another advantage of 2618-T61 over the high silicon pistons is the ability to keep its shape under extreme pressure and high RPM's. "Skirt Shrinking" is not a problem with ROSS Racing Pistons.

The “4032” performance piston alloy has an approximate Silicon content of 11%. This means that it expands from heat less than a piston with no Silicon, but since its eutectic level of Silicon is fully alloyed on a molecular level, this alloy is less brittle and more flexible than a stock Hypereutectic “smog” piston. These pistons can survive mild detonation with less damage than stock pistons.

The “2618” performance piston alloy has less than 2% Silicon and could be described as Hypo (under) eutectic. This alloy is capable of experiencing the most detonation and abuse while suffering the least amount of damage. Pistons made of this alloy are also typically made thicker and heavier because of their most common applications. Because of the higher than normal temperatures these pistons experience in their usual application, and also the low-Silicon content allowing the maximum possible Aluminum heat-expansion, these pistons have their cylinders bored to a very loose cold-fit. This leads to a condition known as “piston slap” which is when the piston rocks in the cylinder, and it causes an audible tapping noise that continues until the engine has warmed to operational temperatures. These engines should not be revved when cold, or excessive scuffing can occur.

4032 pistons will have quieter cold start operations due to their tighter piston to wall clearances compared to 2618 pistons. This is due to the 15% greater thermal expansion seen in the 2618 alloy. 15% may seem like a lot, but do the math. Considering a piston to bore clearance of 2/1000's of an inch, 15% is only .0003". Once the pistons have reached their operating temperature, the noise (piston slap) differences should be nearly identical in volume between the two alloys. 4032 pistons will have reduced oil consumption and longer ring life compared to their 2618 cousins due to their better cold start tolerances. While to many these physical comparisons point towards 4032, you must understand that 2618 pistons, for their slight “defects”, are clearly superior in terms of tensile strength and fatigue endurance to 4032. This is why most piston manufacturers specify the 2618 alloy for use in Subaru (turbocharged) pistons.