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IAG Performance



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

IAG's Stage 4 Tuff Closed Deck EJ25 converted case halves are the most cost-effective short block modification for high horsepower Subaru WRX and STI's. IAG machines the case halves assembled together in our Haas VF-4SS CNC mill 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. The case halves are then CNC machined for our new Fire-Lock head gasket solution that is loosely derived from diesel technology where a counterbore step is machined around each cylinder bore allowing specially sized fire rings to drop into the machined area and head gasket bores. The Stage 4 Tuff case is then drilled and tapped to accept 14mm head studs that combined with the IAG Fire-Lock head gasket set greatly reduces head gasket failure during high cylinder pressure scenarios. IAG’s Closed Deck is suitable for street or race use and has been tested in vehicles producing 1000+ horsepower and over 55psi of boost.

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 uses threaded plugs to provide additional support at the case bolt access positions. This innovative design is just one of the features that set our conversion apart from others on the market. Another innovative feature is 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 inch cast material and the inner inserts from 1” 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 Stage 4 Plus 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 VF-4SS CNC Vertical Machining Center. It is supported from 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. Specially machined, threaded case bolt plugs utilizing a 3/8 inch square drive are installed and torqued into place. Once they are torqued a ½ inch square drive is machined into each to allow coolant to properly flow and ensure that the thickest part of the plug is supporting the weakest area of the case. 

Each deck surface is then machined true and flat using a Poly Crystal Diamond insert. Finally, we engrave each plug for location and indexing. This step ensures that the plugs are installed in the correct position and rotation after the block is assembled.

What's Included:
  • (x1) IAG Stage 4 Tuff Closed Deck converted EJ25 Case Halves (Starts with brand new OEM case 99.5mm bore) *Bore and hone additional and required before final assembly.
  • (x8) Threaded case bolt plugs
  • IAG Fire-Lock receiver groove machining, latest solution to prevent head gasket failure on high horsepower and high boost Subaru's - *IAG Fire-Lock head gaskets required
  • (x1) Machined case to fit 14mm head studs - *Head studs sold separately

Additional Options:
  • (x1) IAG / ARP Spec 14mm head studs, nuts, and washers - $550

Head Gasket Required:
  • *IAG Fire-Lock .051" Head Gaskets with IAG-Spec Alloy Ring Set* PN# IAG-ENG-1740

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

2002-2014 WRX
2004+ STI
2005-2009 Legacy GT
2004-2014 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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