this is a 97mm piston from thumper racing/cp Carrillo making it a "540". bought from DJH. was just inspected by dave and he says everything looks good. the cylinder is an iron sleeved KTM RFS cylinder. the rings and cylinder have 5 hours on them. this will fit many different year ktm RFS motors, 450, 525, between around 2003-2007 as well as the polaris outlaw 450 and 525 and ktm atv xc models 2008-2011. ask any questions prior to purchase. sold as is.