Pulled of the cylinder head on a buddies 99 YZ400 today. Had very low compression and would not start. Plan to do a complete valve job. Have not pulled off the cylinder yet to look at the piston but the bore still looks nice and you can still see the marks from the hone. There was a lot of carbon on top of the piston but it looks good after the carbon was removed. Wondering if we should go ahead and pull off the cylinder and do rings and maybe piston. If the piston measures within spec would you replace it just for protection? Also have a new timing chain ready to go.