Want to push big power from your EJ25?

To produce reliable power, a CNC Closed Deck Modified Block is the only way to go!

An inherent issue with the EJ25 blocks are the cylinders flexing under high power levels, causing a wide range of issues. This is an issue with both the semi-closed deck EJ255 & EJ257 variety, as well as the weaker open-deck blocks. Check out our article on deck designs If you want to know more about open, semi and closed deck block variations. The solution is a Closed Deck Block, which provides support around the top of the bore. Unfortunately, Subaru have never produced a 2.5L Closed Deck Block. Possum Bourne Motorsport have designed and developed a CNC program to close deck EJ25 blocks. PBMS recommend moving to a closed deck 2.5 if targeting in excess of 300kw/400hp at the wheels.


  • Reduced deck flex - improving cylinder head and gasket sealing
  • Reduced liner flex -  reducing liner movement and cracking
  • Improved cylinder support - maintain bore shape and aid compression


We have the CNC program to close deck EJ25 blocks with the following casting numbers. Please contact us if you have another block.

  • Z25L
  • B25C

Note: We have also developed a 2L variation to suit EJ207 semi-closed deck block as found in V7-10 STIs.


  1. Book a CNC Close Deck modification with PBMS
  2. Deliver stripped and cleaned block halves to PBMS
  3. Fit block halves to machining fixture
  4. Machine block for Closed Deck Plate
  5. CNC machine Closed Deck Plate
  6. Insert Closed Deck Plate into block

To complete post modification (Not included in price)

  1. Machine bores to suit pistons
  2. Calculate cylinder deck heights and have decks machined accordingly
  3. Thoroughly clean block halves before assembly

Please note: this is a process that can only be performed with the engine fully disassembled, so we recommend doing this at the time of an engine rebuild. The prices indicated are for the modification to the block halves only and do not include remove, stripping, cleaning, deck surfacing, assembly of the engine or any other components in the photo. PBMS takes no liability for an engine we do not inspect or build from start to finish. We recommend taking measurements and checking the block suitability prior to Close Deck Modification.  



  • EJ25 CNC Closed Deck Modification.
  • Recommend for above 300kw/400hp (At the wheels)
  • Designed to strengthen an EJ25 block for high power reliability.
  • Discuss the suitability of this modification for your engine, please contact us.


SKU 1804
Brand PBMS
Shipping Weight 25.0000kg
Shipping Width 0.500m
Shipping Height 0.500m
Shipping Length 0.500m
Shipping Cubic 0.125000000m3

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