IAG Performance Stage 3 Magnum EJ25 Closed Deck Case - 14mm H.S.M., Fire-Lock, (No Bore & Hone) - For Subaru WRX, STI, LGT, FXT

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SKU IAG-ENG-1818

IAG's Stage 3 Magnum Closed Deck converted case halves were created to be the most affordable and cost-effective short block modification for EJ25 Subaru turbo engines. 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 Magnum 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.

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. 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 (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-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's Included:

  • (x1) IAG Stage 3 Magnum Closed Deck converted EJ25 Case Halves (Starts with brand new OEM case 99.5mm bore) *Bore and hone additional and required before final assembly!
  • (x1) Machined for 14mm Head Stud Conversion (14mm Head Studs sold separately)
  • (x1) 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


Additional Options:

  • (x1) IAG 14mm Inch 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 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.

IAG's Stage 3 Magnum Closed Deck converted case halves were created to be the most affordable and cost-effective short block modification for EJ25 Subaru turbo engines. 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 Magnum 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.

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. 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 (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-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's Included:

  • (x1) IAG Stage 3 Magnum Closed Deck converted EJ25 Case Halves (Starts with brand new OEM case 99.5mm bore) *Bore and hone additional and required before final assembly!
  • (x1) Machined for 14mm Head Stud Conversion (14mm Head Studs sold separately)
  • (x1) 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


Additional Options:

  • (x1) IAG 14mm Inch 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 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.


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