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Old 06-27-2008, 09:50 PM   #1
front runner
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Member#: 121603
Join Date: Jul 2006
Chapter/Region: NWIC
Location: Seattle, Washington
Vehicle:
2001 legacy BH
blue

Default Sti A and B grade pistons?

Does anyone know the difference between A and B grade pistons? To me, they look exactly the same and the search button isn't helping. I have a new set of oem's that I got for cheap and I don't want to put them on the wrong sides.
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Old 06-28-2008, 01:26 AM   #2
morrocco88
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Join Date: Jan 2006
Chapter/Region: NWIC
Location: Eastern WA
Vehicle:
97' LGT 2.3 Dstroke
Rotated BW S256 6speed

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The difference is the actual size. I think Subaru did this to compensate for machining differences in the bores. The cylinder bore size is stamped on the block's front upper surface.

The "B" piston has an outer diameter of 3.6211 to 3.6214 inches

The "A" piston has an outer diameter of 3.6214 to 3.6218

This way you can select an "A" or "B" piston to get the correct piston to sidewall clearance. Measure the bore, do the math, and select the correct piston for that bore.

I'm reading this right out of the FSM for the STI. I bought it for my upcoming sti short block with EJ20G or 2.5 N/A heads motor.

Hope this helps!
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Old 06-28-2008, 01:33 AM   #3
IllNastyImpreza
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DIY project: life
I need more TIME!

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holly **** are you serious? I didn't know they had this. thats actually pretty good of subaru to do that. this way you are not just buying an "off the shelf" piston, and hoping it fits !

BTW, you will get WAY more responces in built motor forum.
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Old 06-28-2008, 03:36 AM   #4
ballitch
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98 cam'd EJ251
Wrx Tranny,slowr than ish

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I just marked mine when I rebuilt my current motor, that way you could compare them to your new ones.


~Josh~
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Old 06-28-2008, 01:44 PM   #5
front runner
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Thanks for the info! I had no idea that what those markings meant. now I can build without that looming over my head
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