Originally Posted by tlfowle
The part numbers I'm finding are 10103AC030 for 04-06 Legacy, 10103AC040 for 07-09 Legacy, 10103AC050 for 08-13 STi.
The differences between 255 and 257 are basically in name only - the year is more important. The two Legacy part#s only differ in pistons. The early block is designed for the larger B25 heads, the latter for the smaller D25 heads. Thus the later pistons have more dish to keep the same compression ratio.
Just my $.02 - I would strongly suggest that if you are going to spring for a new shortblock and have any future performance desires get a block w/ 4032 forged pistons and the bearing clearances blueprinted. Yes, you will spend a little more than new from Subaru but you'll have a better platform to work from for future mods. The 4032 pistons (Mahle, Wossner, Cosworth) are very streetable but will hold up better to performance mods since they are less brittle than the stock cast pistons. Bearing clearances are a crapshoot in even new Subaru blocks - maximum reliability is when all of the clearances are the same.
Sorry for the level of detail - I'm in the middle of building a motor and have been through every possible combination to make sure it is build right.