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IAG Stage 4 CNC Ported Heads Set Stage 4 CNC Ported Heads w/ +1mm GSC Valves W25 Casting (Cams/Lifters Sold Sep)

Engine Cylinder Head Set


IAG Stage 4 CNC Ported Heads w/ +1mm GSC Valves for 08-17 STI W25 Casting.

  • New Head Castings, Fully CNC Ported with +1mm GSC Valves, GSC Beehive Springs
Pricing: $4,599.99

Free ground shipping on all order over $100.00 (some exclusions may apply)


Subaru: WRX STI 13 - 18 STI / Series.HyperBlue / Limited / Base / Tsurugi Edition / Sport / Sport-tech / Launch Edition, Subaru: Impreza 08 - 14 WRX STI

  • Recommendations / Warranty: Assembly and professional installation recommended. Workmanship/parts warranty will only be available for professionally installed cylinder heads.
  • Stage 4 Ported Head Package Includes: (x2) Brand New Subaru OEM W25 Cylinder Head Castings, (x8) GSC 37mm (+1mm) Intake Valves, (x8) GSC 33mm (+1mm) SuperAlloy Exhaust Valves, (x16) GSC Beehive Springs & Retainers, (x16) GSC Manganese-Bronze Valve Guides, (x16) GSC Valve Stem Seals, IAG Performance CNC Porting Multi-Angle Valve Job, Drilling for compatibility with 1/2" or 14mm Head Studs, Cam Clearancing, Ultrasonic Cleaning

IAG's Stage 4 Cylinder Head Package utilizes a CNC porting program for the W25 heads that was developed over the past five years and tested on the most successful high-horsepower Subarus. The ported head package starts with brand new WRX STI W25 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 receive our combustion chamber modification (depending on option choice) in which the shapes of the chamber are machined to reduce compression and improve the burn path. Next, 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. Shortly 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 in-house Serdi 3.0 Valve Guide and Seat Machine to match GSC's +1mm intake and exhaust valves. Then each intake and exhaust seat undertakes 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. 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.


  • IAG-ENG-3040W
  • IAG-ENG-3040W
  • 887753778680
  • IAG Performance
  • New


Shipping Information

  • Item Requires Shipping
  • 72.0 lbs.
  • W16.0000” x H12.0000” x L20.0000”
  • Free ground shipping on all order over $100.00 (some exclusions may apply)

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