ok my friend has a 98 polaris sport 400 and he bought it used and they said they had to rebuild it well it needs a new one and he does not now what size to get it says 675 somthin he syas please help him we need to go riding

 

The stock bore is 3.270 inches, They "probably" put a 20 or 30 over piston in with the rebuild, A short trip to the machine shop can end the guessing game by having them measure the bore and determine what over bore he will need to do next.

 

Depends on what bore he is on now:

675PS: 83mm (STD)
675P2: 83.5mm (.020)
675P4: 84mm (.040)
675P6: 84.5mm (.060)
675P8: 85mm (.080)

From there its on to an HPD big bore, with uses old YZ400/490 pistons.