So I'm in the process of starting to rebuild a engine for the first time the motor is still in the car so I have a little ways to go but My block piston sizes are BB|AB With A being cylinder 2 so it's likely that I just need a hone do I buy replacement stock pistons in that size?

 

First, why are you replacing pistons due to a rod knock?  A rod knock usually starts as a bearing issue and you may need to replace one or more rod but the pistons don't necessarily need replaced.  Especially if you haven't pulled the motor and inspected the pistons/rings.

 

How many miles are on the car?  Have you priced a crank kit yet (crank + bearings), etc...?

So I'm in the process of starting to rebuild a engine for the first time the motor is still in the car so I have a little ways to go but My block piston sizes are BB|AB With A being cylinder 2 so it's likely that I just need a hone do I buy replacement stock pistons in that size?

 

that's a long run on sentence, i can't tell exactly what you're asking, but i'd repeat his reply above - why are you replacing pistons?

Since it's an EJ255 (turbo), it's probably a blown ringland, which is not knock.

 

I'm no expert on this, but 'think' it depends on how badly the cylinder walls are damaged, correct?  They need bored>honed and then measured. Subaru sells .020 and .040 oversized pistons, I believe.

td

Pull the block and inspect the cylinder walls for scoring first. Might need a bore and hone as wtDash said.

 

Subaru sells 0.25 and 0.50mm oversize. Works out to 0.020", that's the max Subaru recommends. If it needs overbore you order all 4 pistons the same size to match the new bore.

it needs new bearings due to excessive blow-by which thinned the oil too much and yes the ringland(s) are cracked so yes I need new bearings

You probably also need a coolant temperature sensor (the one which goes on the coolant crossover pipe).  It can cause a "run rich" situation which thins the oil causing the blow by.  

Just to be "correct", blow by is the result, not the cause of fuel washed oil.

 

Emily

My car was in no way running rich it was running too lean at idle The car has over 202k on it it was burning oil (1 qt every 300 miles)

i'm pretty sure you know this,

but is easier, cheaper, and quicker to find a good used engine and swap it in.

 

but then every one needs a hobby,

so if this is yours,

have at it.

My car was in no way running rich it was running too lean at idle The car has over 202k on it it was burning oil (1 qt every 300 miles)

 

Ah, with that kind of mileage, yes, the blowby can cause the fuel wash. 

BTW, put a new Subaru PCV valve on your new engine too.

yeah the PCV is hardly 8k miles old but I will be replacing it again anyways, the timing pulled was insane.. it would pull so hard you could floor the car and it still would got go any faster until it got over 1800 rpms and it just woke up again.. Some times it would keep pulling timing under 1800 and sometimes not.

 

I will say it was a good lesson to see what a huge pull in timing felt like