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IAG Performance S3 EJ25 Case Half Set Stage 3 EJ25 Closed Deck Case - 1/2" H.S.M. 99.75mm Bore & Hone

Engine Long Block


IAG Stage 3 EJ25 Closed Deck Case - 1/2" H.S.M. 99.75mm Bore & Hone - for Subaru WRX, STI, LGT, FXT.

  • Designed for those wanting to assemble their own engine
  • IAG's Stage 3 Closed Deck is suitable for street or race use.
Pricing: $2,749.99

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


Subaru: Legacy 05 - 09 GT spec.B / GT, Subaru: WRX STI 15 - 21, Subaru: Forester 04 - 13 XT Premium / XT / XT Limited, Subaru: Impreza 02 - 14 WRX STI / WRX

  • Case Includes: (x1) IAG Stage 3 Closed Deck converted EJ25 Case Halves (Starts with brand new OEM case), Bored & Honed to 99.75mm, Machined for 1/2 Inch Head Stud Conversion (1/2 Inch Head Studs sold separately)
  • Head Gasket Recommendations: IAG Recommends JE Pro Seal .039 inch head gaskets: JE PN# SB1001-039 are for use with S20, B25 & D25 Cylinder Head Castings (04-06 STI, 06-14 WRX, 04-13 FXT, 05-12 LGT, 02-05 WRX), JE PN# SB1003-039 are for use with V25, W25 & N25 Cylinder Head Casting (2007-20 STI)
  • If using on 2002-05 Subaru EJ20 2.0L WRX additional modifications and parts are required. Attention: Special note about using ARP Case Bolts - IAG recommends line honing the case when using ARP Case Bolts to remove any distortion created by the hardware. IAG installs ARP CASE BOLTS with 30W engine oil using OEM SUBARU torque specs and procedures. Other torque specs and lubricants may lead to case distortion damage.
  • Please provide the following when ordering: The Piston Manufacturer, Your Desired Piston To Wall Clearance
  • The IAG Closed Deck starts with a new OEM EJ25 bare case. The case is loaded and fixtured to a 4th axis rotary indexer in our 2015 Haas VF-4SS CNC Vertical Machining Center. It is supported by the main journals using a precision ground 2-inch bar and tailstock which ensures the datum is the main journals. This method gives us a true reference point to perform all machining operations. A Rennishaw probe is utilized to check that the case is fixtured true and once confirmed the machining process begins. Once the case is fixtured, the first step is to mill out 12 pockets for CNC inserts. Rather than attempting to install inserts into an inconsistent cast case, the pockets are milled out to exact dimensions creating a consistent area that accepts our precision inserts. Once the pockets are machined, the case is removed from the mill and placed into a block oven at a low temperature. Our pre-machined closed-deck inserts are simultaneously chilled using dry ice. When the case and inserts have reached our desired temperature, they are moved into our install station, which holds the block and lightly presses the inserts into position on one side of the deck surface. This process is repeated on the other side and then the partially completed case is moved to a cooling rack. In the next step of our process, the case is loaded and fixtured back into the VF-4SS after reaching room temperature. During this stage, the flanges left from machining the inserts are milled close to the case deck surface. Features such as coolant passages and case bolt access positions are machined in preparation for the final stages of our conversion process. Each deck surface is then machined true and flat using a Poly Crystal Diamond insert. Finally, the case is 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 tighter for a quieter running engine.

IAG's Closed Deck converted case halves were created to be the most affordable and cost-effective short block modification for EJ25 Subaru turbo engines. The deck surface is machined to accept specially CNC'd cast inserts that strengthen the weakest point of the OEM case at the upper sleeve. The case halves are then CNC drilled and tapped to accept 1/2 inch head studs that greatly reduce headgasket failure during high cylinder pressure scenarios. The case is bored and honed to 99.75mm in our Sunnen SV-10 that allows you to set a piston to wall clearance tighter for a quieter engine. IAG's Closed Deck is suitable for street or race use. The factory Subaru EJ25 case is referred to as a semi-open (or semi-closed) deck. The block is essentially an open deck with 3 narrow supports tying in the main case to the tops of the sleeves. This feature was a slight improvement over the EJ20 case, which was a completely open deck with no support. The purpose of the deck supports is to keep the sleeve from moving. While conceptually this is an improvement, the reality is that it simply isn't enough for a high horsepower Subaru. The weakest point of the sleeve is still left open and unaddressed. Our machining process converts the semi-open deck to a complete closed deck. This design features the use of cast insert material that closely matches the metallurgy of the aluminum case material in a factory Subaru EJ engine block. By using a similar cast material, we ensure that during expansion and contraction the materials grow and shrink at similar rates, which is vital for maintaining roundness within the top of the bore. The outer inserts are machined from 3/4 inch cast material and the inner inserts from 1-inch cast material. Once the deck is surfaced, the result is a true 1/2 inch finished outer insert thickness and true 3/4 inch finished inner insert thickness. The IAG Closed Deck conversion is performed 100% in-house by our engineers and experts using state-of-the-art equipment.


  • IAG-ENG-1813
  • IAG-ENG-1813
  • 887753778253
  • IAG Performance
  • New


Shipping Information

  • Item Requires Shipping
  • 70.0 lbs.
  • W70.0000” x H16.0000” x L19.0000”
  • Free ground shipping on all order over $100.00 (some exclusions may apply)

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