IAG's Stage 4 Cylinder Head Package utilizes a CNC porting program for the D25 heads that was developed over the past five years and tested on the most successful high-horsepower Subaru's. The ported head package starts with brand new D25 WRX / Forester XT / Legacy GT OEM head castings directly from Subaru of America. The bare castings are visually inspected then fixtured into our in-house Centroid 5-axis CNC Machine, one of the most advanced precision engine machining equipment in the world. The Centroid CNC ports the intake and hard to reach exhaust "dog-leg" port runner. The benefit of CNC porting is not only an efficient way of machining the heads it also offers consistent port results that are hard to achieve by hand.

Once the Centroid completes the port process the heads undergo a cam clearance machining process that removes material from the lifter area where large duration cam profiles are known to contact the castings. Short after the heads are drilled to fit larger 1/2" or 14mm head studs which are commonly used in high horsepower builds. Next, the OEM guides are pressed out and replaced in favor of GSC Manganese-Bronze valve guides that are shaped to improve airflow.

Once all the new intake and exhaust guides are in place they are diamond honed in our honing station. The heads are then fixtured in our Serdi 3.0 Valve Guide and Seat Machine to cut all intake and exhaust seat with multi-angle valve job that suits the flow characteristics of GSC's +1mm valves. At this point, the heads decks are resurfaced using our TS-3000 BlockMate that cuts the aluminum to a smooth even finish. Next, the heads are ultrasonically cleaned to remove debris and oils from the machining process. Finally, the heads are reassembled using GSC Beehive Valve Springs and Retainers along with GSC Viton stem seals.

Recommendations / Warranty:

• Assembly and professional installation recommended.
• Workmanship/parts warranty will only be available for professionally installed cylinder heads.