IAG Performance Stage 3 EJ25 Closed Deck Case - 11mm H.S.M., (No Bore / Hone) - For Subaru WRX, STI, FXT & LGT

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SKU IAG-ENG-1810
IAG's Stage 3 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. IAG's Stage 3 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's included:

  • (x1) IAG's Stage 3 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 OEM 11mm Head Studs


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

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.

IAG's Stage 3 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. IAG's Stage 3 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's included:

  • (x1) IAG's Stage 3 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 OEM 11mm Head Studs


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

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.


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