Help !, i have that engine knock thing in the morning,goes after 10 mins of warming up ,then sounds real smooth,i read about replacing the 4 piston,then read you should replace all 4,so given that it is a piston knock when cold,should i replace all 4 @ £141 for cosworth forged pistons,(dont no cost of fiiting though) or have i got it all wrong and it doesnt need doing or the knock is somthing completely different? like the name sujests ! virtgin !

Blue 5 door imprezza turb year 00,owned for just 3 weeks,new at modification of cars,but not bikes ! added clear lights front,side only,magnex full de-cat, front to back, jetex air filter(yer ive been told,buy a itg or something !

 

I think you are describing piston slap, not knock.

There is a sound file knocking about with slap demonstrated.

Steve

 

If it is "slap" rather than knock, you can simply replace number 4 piston, preferrably with gen item, otherwise, if using another make of piston, then yes, you will need to replace all 4 to ensure correct compression pressures with the identical pistons.
easiest way is to prevent that cylinder from firing, ie take plug lead off and then run, if knock goes, then piston slap, if still there, then maybe crank or big end bearing.