Stage X Piston Set - Low Compression WRX & STI 2006-2013 (V9+) EJ25

Forged piston set for EJ25 engines in the WRX and STI (2006-2013, V9+).

Why forged?

Forging is still the best way to make a piston, both from a production point of view and for strength. A piston machined from solid is marginally better in a few areas, but the strength benefits of a forging far outweigh them.

A correctly forged piston has an excellent grain structure. A fully machined piston has no true grain flow, because the material is used as it is cut from the bar. Forging creates optimal alignment within the material and, combined with proper heat treatment, this is what gives the piston its strength.

Accuracy and attention to detail

• Most of the machining that turns a forging into a piston is done on CNC (Computer Numerical Control) machines.
• Pistons can expand by as much as 10 microns in sunlight, so they are measured at a constant 20.5°C. This way tolerances can be checked anywhere in the world, provided the same 20.5°C conditions are replicated.
• The skirt is designed and machined to give a profile that works most efficiently at full engine working temperature. Skirt profiles are not round but elliptical, and barrelled along their length.
• The skirt is machined on extremely accurate machines that hold repeatability down to 5 microns.
• Every piston is individually weighed and batched to within 0.5 grams, so a set of four has a deviation of no more than 2.0 grams. Sets of four with only 0.1 gram deviation are not unusual.