IAG Stage 4 Ported Heads with +1mm GSC Valves & Cams: IAG's Stage 4 Cylinder Head Package utilizes a CNC porting program for the S20 heads that was developed over the past 5 years and tested on the most successful high-horsepower Subarus. The ported head package starts with brand new S20 WRX OEM head castings directly from Subaru. The 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. CNC porting offers consistent port results every time. Next the heads receive our combustion chamber modification in which the shape of the chambers are machined to match those of an STI head. The heads then undergo a cam clearance machining process that removes material from the lifter area where large duration cam profiles are known to contact the castings. The heads are drilled to fit larger 1/2" or 14mm head studs commonly used in high horsepower builds. Next, the OEM guides are pressed out and replaced with GSC Manganese-Bronze valve guides that are shaped to improve airflow. Once 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 seats with a 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. After that the heads are ultrasonically cleaned to remove debris and oils from the machining process. The heads are then reassembled using GSC Billet S3 camshafts. These cams are intended for vehicles making 700+ wheel horsepower. In final assembly the cams are matched to GSC Beehive Valve Springs and Retainers along with GSC Viton stem seals then bench-lashed to a tolerance that is close to final spec.