On a zetec se engine do the pistons rock much at tbc as I have a new engine I'm building and I'm worried

 

You will see a tiny amount of movement at TDC, as the con rod is on the rock, but so long as bore clearance was done correctly nothing to worry about.
tabetha

 

And how would I know the bore clearance are correct?

 

You would measure the bore clearances before final assembly.

 

Has the engine been rebored ?
If the engine has been rebored, it should have been bored to the clearance specified by the maker of the pistons you used by the machine shop.
If it doesn't have new pistons and there is any lip at the top of the bore where the rings don't come up far enough to wear, then it needs a rebore, although some would just re ring the pistons.
The vast majority of the wear does NOT occur at the top but midway down the bore on the thrust side, this is where a machine shop would drop a bore gauge down and measure at, but if it's got wear at the top it has certainly got wear at midway point.
Re ringing the pistons is a stop gap measure imo, as the wear is still there.
Any re boring should have been done with a stress plate also when done, I would not accept a block bored without, but a lot still do it!!
tabetha

 

carefull with the rings on those as well if replacing ive seen thicker mig wire lol