I have just torn down my Polaris trail boss 250 and it needs a top end the cyl is 72.5678mm and the old piston is 72.2376mm so I'm trying to figure out the right size piston for the new one and am completely lost as to what to order

 

Stock cyl is 72mm (2.836)
72.5678 mm on your cylinder is (2.857)
72.50 mm on Wiseco (which is .020 over stock) OR (2.854) Looks like it's been bored .50mm(.020) over already and has worn down slightly more. Disregard the piston as you have to go by the cylinder measurement. Looks like you need to go to 73mm to clean the bore up if you use a Wiseco which is highly suggested.Product Search Engine For All Products - Wiseco Piston Inc.
Also the 250 piston to cylinder clearance in the manual shows .0011-.0021 which is tight when you have it bored and finish honed. Plus by your measurements the piston to cylinder difference you have now is .3302mm or .013 clearance which is way over the .006 maximum service limit.

 

It already being bored over would explain the marking 5w-50 on the old piston and the bore it self looks and feels good so couldn't I just use my finish honer and put a .020 piston in

 

.020 is 50 mm which is the same size as your old piston!! Even if you got the same size new piston (50 mm) if the piston/cylinder clearance exceeds .005 it would be a waste of time and money! Pistons do wear down along with cylinders,but if you're going this far, bore it to 1 mm over (73 mm) so that any out of round, egg shape and rough places can be removed from the cylinder to match the new piston. Again a new piston should be set up within clearances as I mentioned earlier. I usually tried for.0015 final clearance fit after a final honing. Clearance is measure from the bottom of the piston skirt to the bottom of the cylinder sleeve.Have the new piston in hand before you have the cylinder bored.