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Old 12-07-2003, 11:26 AM   #2
Member#: 456
Join Date: Oct 1999
Chapter/Region: NWIC
Location: Redmond, WA, USA
2008 Forester XT
Steel Gray Metallic


Subaru Australia Forester XT description:

The pistons feature improved high tensile strength aluminium alloy (AC8A->UAC12H) along with optimised piston crown shape for improved fatigue resistance.

Subaru Media info for STi:

In the section "300 Horsepower and ..." it says the STi has forged aluminum alloy pistons.


SOA STi News Release on

The 2.5-liter horizontally opposed ("boxer") four-cylinder engine is based on a specially reinforced "semi-closed deck" engine block with forged aluminum alloy pistons, forged high-carbon steel connecting rods and sodium-filled exhaust valves.

Car and Driver Forester XT review:

There's a patented new trick to this engine, too. Ichikawa-san's engineers found that the aluminum rear main bearing cap was letting the crankshaft wobble a bit, opening up the bearing clearance over time and generating a rattling noise. So they invented a new kind of main bearing cap that has a shaped sintered-steel core and an aluminum wrapper that does the job with two conventional bolts. The 2.5-liter XT turbo engine also gets a new semi-closed-deck block, heavy-duty connecting rods, forged pistons, and a drive-by-wire throttle setup.

Most of these from Jon in CT.

From Specul8r:
Here's my question to SOA:


I purchased a 2004 STi last July, and have one quick question. I see
information on your website that states that the STi comes with forged
aluminum pistons. A well-know engine performance company disputes that
they are forged. My question to Subaru: Are the 2004 STi pistons
forged aluminum?

Here's their response:
Thank you for visiting the Subaru Web site and for your inquiry. The
2004 WRX STi is equipped with forged steel pistons and forged aluminum
conrods and crank.

Thanks for the opportunity to be of assistance. If you need any future
assistance, please feel free to contact us again.

Info posted by APS Australia:

Other random stuff:

From STi specs:

(it is silent on pistons)

Horizontally opposed 4-cylinder intercooled high-boost turbo with die-cast aluminum-alloy block and aluminum-alloy cylinder heads. Intercooler has driver-controlled manual water spray cooler with low-fluid indicator.
News on, STi specs (which is alongside the core technology doc, but is silent on pistons):


Part numbers for USDM STi:
12006AC390 (std grade A)
12006AC400 (std grade B)
12006AC410 (OS 0.25mm)
12006AC420 (OS 0.5mm)
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Last edited by ANZAC_1915; 12-07-2003 at 11:51 AM.
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