Stripped by ver 4 wrx dueto knocking/slap and the piatons seem very loose in the barrel you can rock them quite alot at tdc. Can anyone tell me the usualable bore size/piston diametre? Thanks

 

Standard bore size is 92.00mm and standard piston to bore clearance is 0.0015"( 0.03mm)

You will probably find that the skirts of the pistons on cylinders 2 and 4 ( passenger side )
will scuffed/bruised and normally the cause of the piston slap you can hear from cold.
Quite common on the Version 3~4 engines.

Mick