IAG Performance Stage 4 Extreme EJ25 Closed Deck Case - 14mm H.S.M., Fire-Lock, Pinned + Line Honed, ARP Case Bolts, (No Bore / Hone) - For Subaru WRX, STI, LGT, FXT

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SKU IAG-ENG-1828

The Stage 4 Extreme is IAG's strongest and most prepared EJ25 Closed Deck offering. 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. 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 Extreme case is CNC drilled and tapped to fit 14mm head studs. Utilizing the new Fire-Lock gasket solution along with larger IAG / ARP 14mm head studs, nuts, and washers reduce the chances of the heads lifting under extreme boost conditions. Finally, ARP case bolts are installed and torqued before line honing the completed case in our in-house Sunnen CH-100 machine. 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's Included:

  • (x1) IAG Stage 4 Extreme Closed Deck converted EJ25 Case (Starts as brand new OEM Case 99.5mm)*Bore and hone additional and required before final assembly!
  • (x8) Threaded case bolt plugs
  • (x1) Machined for 9.5mm pinned mains
  • (x1) ARP Case Bolt Set
  • (x1) Line Honed Mains
  • Fire-Lock - 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 for 14mm Head Studs (14mm Head Studs sold separately)


Additional Options:

  • (x1) IAG / ARP 14mm Head Stud Set


Head Gasket Required:

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


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

 PDF Closed Deck Install Guide for 1/2" & 14mm Head Studs

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 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.

The Stage 4 Extreme is IAG's strongest and most prepared EJ25 Closed Deck offering. 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. 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 Extreme case is CNC drilled and tapped to fit 14mm head studs. Utilizing the new Fire-Lock gasket solution along with larger IAG / ARP 14mm head studs, nuts, and washers reduce the chances of the heads lifting under extreme boost conditions. Finally, ARP case bolts are installed and torqued before line honing the completed case in our in-house Sunnen CH-100 machine. 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's Included:

  • (x1) IAG Stage 4 Extreme Closed Deck converted EJ25 Case (Starts as brand new OEM Case 99.5mm)*Bore and hone additional and required before final assembly!
  • (x8) Threaded case bolt plugs
  • (x1) Machined for 9.5mm pinned mains
  • (x1) ARP Case Bolt Set
  • (x1) Line Honed Mains
  • Fire-Lock - 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 for 14mm Head Studs (14mm Head Studs sold separately)


Additional Options:

  • (x1) IAG / ARP 14mm Head Stud Set


Head Gasket Required:

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


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

 PDF Closed Deck Install Guide for 1/2" & 14mm Head Studs

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 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.


This product can expose you to chemicals including Lead, which is known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov