750 Commando bore size 73mm

0.52mm =0.0205" or 20 thou so you need +20thou pistons,. You need 5 thou clearance so it may take a hone and this may remove the scrape.

Measure you new piston before the hone.

Previously wayne_tobin wrote:

Thanks John 20 over is not to bad,i will take your advice on the hone.

Wayne

Pistons are normally made with built-in clearance and cam ground, ie not perfectly round.measured at ninety degrees to the gudeon pin about half inch up from the skirt

The bore size you mentioned is as John stated plus .020" so they are what you need with only the lightest of hones just to glaze-bust the bore and take off any high spots to help the new rings bed-in

Steve

Hi Wayne,

Before you commit to buying new pistons, it would be well worth getting the bores measured properly to establish exactly how worn they are. With pistons the price they are, it would be folly to put new pistons into a worn bore. They may look ok but a nice accurate bore on a new set of pistons will be well worth the effort and cost. The peace of mind of knowing that you have done the best job is worth it.