Look at the VIN number of the bikes in question. 2011 models will have a letter B as the 10th character, 2010's have a letter A, 2009 has a 9. The engine serial will have a B as the 7th digit for a 2011, A for 2010, 9 for 2009. The next character is the month of manufacture of the engine. A=Jan, B=Feb etc. Followed by the same serial as the frame if the bike's 'numbers' match.