EJ255 vs. EJ257: Which Block Is Right for Your Build?

They share a displacement, a bore, a stroke, and a bolt pattern. They are not the same block. Here is what actually differs, and which one belongs in your car.

Same displacement, different jobs

The EJ255 powered the WRX (2006+), the Legacy GT, and the Forester XT. The EJ257 powered the WRX STI. Both are 2.5-liter turbocharged EJ-series flat-fours: 99.5mm bore, 79mm stroke, 2,457cc.

Subaru built the 257 for the STI's higher output and harder duty cycle. The 255 was built for the WRX's price point and daily-driver role. That difference in intent shows up in the hardware.

The real differences

Deck design. The EJ257 uses a semi-closed deck. The block casting ties the cylinder walls into the deck at the top, which resists cylinder distortion under high cylinder pressure. Most EJ255 blocks are open-deck castings — fine for stock and moderate boost, less rigid when you push hard.

Factory internals. The 257 left the factory with stronger internals than the 255. Pistons, rods, and bearings were all spec'd for the STI's output. The 255's internals were spec'd for the WRX.

Compression and heads. Compression ratios and cylinder head designs vary across model years for both engines, so check the specific block you're buying rather than assuming. The headline is simple: the 257 was the performance motor, the 255 was the volume motor.

What to verify before you buy

Block castings get mislabeled. If you're buying a used block, verify the casting marks and the engine code stamped on the block — not the seller's listing title. A 255 with STI heads bolted on is still a 255 block.

Also confirm which heads you're pairing with the block. Combustion chamber volumes differ across EJ heads, and chamber volume plus piston dish sets your actual compression ratio. The wrong head/block combination gives you the wrong compression. If you're not sure, ask before you buy — it's a five-minute question that prevents a very expensive mistake.

The myth: "They're identical"

This one survives because the blocks look alike and bolt up alike. Early internet lore held that the 257 was just a 255 with STI heads. It was never that simple. The deck casting and the factory internals are genuinely different, and at high boost levels the difference matters.

That said, the myth has a kernel of truth worth knowing: if you are tearing the block down and installing forged internals anyway, the factory internals stop mattering. What remains is the casting — and the semi-closed deck of the 257 is the better foundation for a high-boost build.

Which one should you buy?

Running near stock or a mild tune: match your car. STI gets a 257, WRX gets a 255. Factory engineering already answered this question.

Building for real power: start with the 257. The semi-closed deck and stronger starting point are worth it when boost goes up.

Full forged build: buy on price and availability. You're replacing the internals either way. The 257 casting is still preferable, but a built 255 is a proven combination.

One more consideration: the 255 is typically the cheaper block. If your build plan already includes forged pistons and rods, paying the 257 premium for factory internals you're about to remove doesn't make sense.

We stock new OEM short blocks for both — the EJ257 and the EJ255 — as well as built versions of each with forged internals for higher-output applications.

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