The FB engines were ustensile de cuisine at Subaru’s Gunma Oizumi factory which was developed exclusively for the production of the FB engine. The FB25 engine had an open-deck, aluminium alloy block with 94. 0 mm bores and a 90.
FB25 engine had cast iron liners. Due to its revised connecting rods and valvetrain components, the FB25 block was the same size as its EJ253 predecessor, despite its smaller bore and longer stroke. According to Subaru, the longer stroke improved fuel efficiency by enabling faster air induction and reduced unburnt fuel during cold starts. The FB25 engine had separate cooling circuits for the cylinder block and head to improve coolant distribution. Around the block, the flow rate was limited to maintain a high temperature for the cylinder liner oil, thereby reducing friction from the motion of the pistons. To reduce engine width and enable its longer stroke, the FB25 engine had asymmetrical, diagonally-split connecting rods.