IAG STAGE 3 EXTREME EJ25 CLOSED DECK CASE - 1/2" H.S.M., O-RINGED, (NO BORE / HONE) - FOR SUBARU WRX, STI, FXT & LGT
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IAG STAGE 3 EXTREME EJ25 CLOSED DECK CASE - 1/2" H.S.M., O-RINGED, (NO BORE / HONE) - FOR SUBARU WRX, STI, FXT & LGT

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IAG-ENG-1814
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IAG STAGE 3 EXTREME EJ25 CLOSED DECK CASE - 1/2" H.S.M., O-RINGED, (NO BORE / HONE) - FOR SUBARU WRX, STI, FXT & LGT
Please Note: Overbore must be performed (99.75mm is the preferred bore) before assembly!

IAG's Stage 3 Extreme Closed Deck converted case halves were created to be the most affordable, cost-effective short block modification for the Subaru EJ25 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 machined with O-ring receiver grooves around the bore surface as well as drilled and tapped to accept 1/2" head studs that greatly reduce head gasket failure during high cylinder pressure scenarios. IAG’s Stage 3 Extreme Closed Deck is suitable for street or race use. 


What is a Closed Deck Block and why do I need it? 

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. 

What is unique about IAG’s Closed Deck process?

Our machining process converts the semi-open deck to a complete closed deck and 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 (rather than billet), 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 ¾” cast material and the inner inserts from 1-inch cast material. Once the deck is surfaced, the result is a true ½ inch finished outer insert thickness and true ¾ inch finished inner insert thickness. 

What is IAG’s Closed Deck Process? 

The IAG Closed Deck conversion is performed 100% in-house by our engineers and experts using state-of-the-art equipment. 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 VF4-SS 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. Finally, each deck surface is machined true and flat using a Poly Crystal Diamond insert. 


What's included:

  • (x1) IAG's Stage 3 Extreme Closed Deck Converted Case Halves (Using brand new EJ25 Case / Stock Bore 99.5mm) *Bore and hone additional and required before final assembly!
  • (x1) Machined for 1/2" Inch Head Stud conversion (1/2" Head Studs sold separately)
  • (x1) Machined and O-ringed Deck Surface

Additional Options:

  • (x1) IAG 1/2" Inch Head Stud Set (PN# IAG-ENG-1700) - Available 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 (2007-19 STI)

    If using on 2002-05 Subaru EJ20 2.0L WRX additional modifications and parts are required.

    Fitment-
    2002-2005 WRX Require Mods
    2006-2014 WRX
    2004+ STI
    2005-2009 Legacy GT
    2004-2013 Forester XT

    Please Note: Overbore must be performed (99.75mm is the preferred bore) before assembly!

    *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.

    *Pictures used for marketing purposes only, actual product may vary 
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