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IAG Performance S4T EJ25 Case Half Set Stage 4 Tuff EJ25 Closed Deck Case w/Plugs, 14mm HSM, Fire-Lock, 99.75 Bore/Hone

Engine Block


IAG Stage 4 Tuff EJ25 Closed Deck Case with Plugs - 14mm H.S.M. Fire-Lock, 99.75mm Bore & Hone - for Subaru WRX, STI, LGT, FXT.

  • Designed for those wanting to assemble their own engine
  • IAG's Stage 4 Closed Deck is suitable for street or race use.
Pricing: $3,299.99

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

  • Attention: Special note about using ARP Case Bolts - 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.
  • Case Includes: (x1) IAG Stage 4 Tuff Closed Deck converted EJ25 Case (Starts as brand new OEM Case), (x8) Threaded case bolt plugs, Bored & Honed to 99.75mm, IAG Fire-Lock receiver groove machining - IAG Fire-Lock head gaskets required, Machined for 14mm Head Studs (14mm Head Studs sold separately)
  • 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.
  • 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 1/2 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.
  • 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 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 3/4 inch cast material and the inner inserts from 1" 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.
  • 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 at 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.

IAG Stage 4 Tuff Closed Deck EJ25 converted case halves are one of IAG's strongest and most prepared EJ25 Closed Deck offering for high horsepower Subaru WRX and STIs. 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. Next, the short block is Deck Plate bored and honed to 99.75mm in our in-house Sunnen SV-30 the most advanced automotive cylinder hone available. IAG's in-house honing process is able to set custom piston to wall clearances to best fit your needs. 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.


  • IAG-ENG-1826
  • IAG-ENG-1826
  • 887753778550
  • 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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