the pista 2004 comes stock with a vp aheadset hs, stack height=25.5mm
chrisking nothread set stack height=28mm

so, you've got 2.5mm difference. i have also heard that if the steer tube
reaches the middle of the top stem bolt, then you should be ok. so, if you
have more than 2.5mm of play, then you should be fine.

edit: i made the assumption that the vp headset is the cheapest one on the
vp website (which is where i got the stack height). its possible that the
vp headset on the pista is a different one that is only available built on
stock bikes. ...so double check which vp headset it is.