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IAG Stage 4 Extreme EJ25 Subaru Closed Deck Short Block (WRX, STI, LGT, FXT)


  • $ 5,999.99
  • Save $ 30000

The IAG Stage 4 Extreme Short Block starts with brand new Subaru 2.5L EJ25 engine case halves. The case halves are split and the main webs are CNC machined in our Haas VF-4SS CNC mill for 9.5mm pins to reduce distortion and movement of the case in extreme conditions. The case halves are then reassembled and IAG machines the case to accept the IAG CLOSED Deck Inserts that transform the block into a solid deck. The unique inserts feature small passages that allow needed coolant flow to the deck surface. Next, the case is CNC drilled and tapped to fit 14mm head studs. Utilizing larger IAG / ARP 14mm head studs, nuts, and washers reduces the chances of the heads lifting under extreme boost conditions. The short block is then Deck Plate bored and honed to 99.75mm in our in-house Sunnen SV-10. IAG‘s in-house honing process is able to set custom piston to wall clearances to best fit your needs. The modified case halves are then assembled using a new 2017+ Subaru 2.5L nitride treated STI crankshaft (or Manley Billet add $1300) that is matched to ACL or King race main bearings. Custom IAG TRI-BEAM +2mm connecting rods by Manley and ARP2000 rod bolts are utilized along with ACL or King race rod bearings. The case halves are sealed with OEM sealant and ARP main bolts. IAG Spec 99.75mm Custom Forged Pistons by Manley with .210 inch wrist pins are chosen to finish the Stage 4 Extreme short block. IAG‘s custom pistons feature a lightweight design while increasing strength where you need it and are designed to work exclusively with the IAG +2mm Tri-Beam connecting rods. The wrist pin is short and is made from strong tool steel. The pin piers are profiled to increase pin boss strut. Finally, IAG‘s pistons feature a 25% larger oil ring for reduced oil consumption and added gas ports that aid ring sealing in high boost conditions. 

Stage 4 Extreme Closed Deck Includes:

  • Case Halves - Converted to Closed Deck from brand new OEM Subaru case halves, bored and honed to 99.75mm.
  • Pinned Mains - Machine case for 9.5mm pinned mains and lined honed.
  • Head Studs - Machined for 14mm Head Studs. *Drilling the head gaskets to clear the larger 14mm head studs is required for fitment.
  • Pistons - IAG Spec 99.75mm 2618 Forged Pistons with .210 inch wrist pins,
  • Connecting Rods - Manley Tri-Beam, ARP2000 Rod Bolts.
  • Rod / Main Bearings - ACL or King Race Bearings.
  • Crankshaft - Standard New Subaru Nitride Treated 2017+ STI Crankshaft.
  • Case / Main Bolts - ARP Main Bolts.

Additional Options Available:

  • Crank - Manley Billet Turbo Tuff Crankshaft - $1300
  • Oil Pump - Subaru JDM 12mm High Volume Oil Pump - $219, or 11mm Subaru Oil Pump - $194.95
  • Rod Bolts - 625+ ARP rod bolts - $200
  • Head Studs - IAG / ARP Spec 14mm head studs, nuts, and washers - $550 *Drilling the head gaskets to clear the larger 14mm head studs is required for fitment.
  • Head Gaskets - IAG Cooper Fire Ring .051" 100mm Head Gasket made for high boost and high horsepower applications with 14mm head studs - $219.98, IAG / JE Pro Seal .039 inch Head Gasket made specifically for 14mm Head Studs - $159.98, Cosworth 1.1mm Head Gaskets modified for 14mm Head Studs - $225
  • Hardware / Seal Option - $105.99 (Includes: Wrist pin plugs, washers, separator cover, bolts, etc. In case you don’t want to transfer from the original engine).

Compression Ratio:

  • 8.5:1 on D Cylinder Heads (EJ255)
  • 8.2:1 on B, V and W EJ257 Heads with standard thickness gasket.

Head Gasket Recommendations:

  • *IAG Strongly Recommends IAG Cooper Fire Ring .051", 100mm head gasket for high horsepower, high boost applications
  • *IAG Recommends JE Pro Seal .039 inch head gaskets for B and D casting cylinder heads.
  • *IAG Recommends Cosworth 1.1mm head gaskets for all V and W casting cylinder heads.

The IAG Stage 4 Extreme Short Block has power output rating of 1000BHP.

*All IAG short blocks are balanced and blueprinted. 

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