Subaru Genuine STI EJ257 JDM Type RA Spec 2.5L OEM Shortblock

This is a genuine Subaru EJ257 2.5 litre Type RA Spec shortblock. This shortblock is standard equipment on the final edition 2021 VAF STi. The 2.5 litre block makes a great upgrade for all WRX models, since there is no replacement for displacement. The Australian STi block features a semi-closed deck for increased strength. This motor is physically a bolt-in for all WRX and STi models (WRXs will require ECU tuning, bigger injectors, etc.).

The Type RA Spec is essentially the same casting as previous EJ257 short blocks and features the same nitrided crankshaft. The conrods also do not change. The only difference appears to be with the pistons:

Upon initial impression, the Type RA and STI piston are both machined cast pistons. The dish volume measured out very close to each other and the valve reliefs are the same depth. Unfortunately the Type RA does not have forged pistons as some had hoped for, although we haven't ruled out the possibility that the Type RA pistons are made from a different alloy that is less susceptible to fracturing (more ductile), as they are supplied by Hitachi rather than Aisin. The crown on the Type RA pistons appears to be 1 - 2mm thicker than the STI piston as well, which is hard to discern from the picture.

From the side profile you can see a few differences. First, the coating at the top ringland is extended to the crown on the earlier STI pistons. This is likely due to manufacturing differences (less tooling / different tooling) rather than any specific performance difference. Second, there is considerably more material supporting the wrist pin on the Type RA piston.

Compression height, ring position, and other dimensions appear to be the same. Meaning, there are no changes to the ringlands in position or thickness. Top ring gaps were 0.010", 0.010", 0.011", 0.011". and lower ring gaps were 0.014", 0.014", 0.014" 0.015".

From the bottom you can see how much material was added to the wrist pin area (in red) as well as the reduction in the large center support (in green). The Type RA design is superior for piston rigidity since it reduces the amount of cantilever, as well as giving better support to the wrist pin itself.

Earlier STI pistons weigh in at 414 grams while the Type RA pistons weigh 424 grams. The extra 10 grams of weight is not a big deal when you consider the improvements made to the piston design. For comparison, forged pistons for the EJ257 typically weigh in the 370 - 400 gram range depending on bore size and design. The wrist pins weighed the same for both motors.

NOTE:

1. THIS IS NOT A COMPLETE MOTOR: This is a fully assembled bottom end, which consists of a block, crank, rods and pistons only. You will need to reuse your stock cylinder heads or purchase replacements.
2. The OEM 2.0L WRX heads will bolt-on for to the 2.5L short block, but there will be a change in compression.
3. For high-powered applications, we recommend replacing the cast OEM pistons in this block with Mahle Forged OEM-size units.