I've incorporated revisions and re-posting after site changes. 2003 WR450F First Year The '03 was heavily based on the 400/426 but the basic engine was considerably lighter (discounting the electric starter) Almost no parts interchange with older models except for wheels, forks, and triple clamps. First year for the side access airbox. The biggest change for the '03 was obviously the electric starter. Automatic compression release DC electrical system (requiring the use of a WR specific CDI) Redesigned seat/tank that more closely match the YZ profile. 2004 WR450F changes New cast Aluminum Kickstand New Internal suspension changes New crank, flywheel, and starter drive parts. There was also a change to how the starter ring gear was located on the crank. '03 models had a washer that locateded on a machined step on the flywheel. '04+ machines did away with this and use a special bolt on the camchain buffer to limit movement of the starter ring. New Kickstarter shaft. Diameter was increased from 18mm to 20mm. New Clutch cover. Changed to accomidate the larger shaft. Torque limiting idler gear assembly for the starter. This was the fix for the infamous "shearing of the woodruff key" problem that plagued the 2003 bikes. The 2004 idle gear assy can be easily installed on the 2003s so if you are looking at buying a 2003 model, make sure it has this upgrade. 2005 WR450F Changes New Lower Seat Height New 48mm Kayaba AOSS (Air Oil Separate System) fork that separates the air and the oil. The 2005 did not have AOSS forks. They were new forks with 48mm stanchion tubes. The forks internally were nearly identical to previous models, using an open cartidge. New triple clamps for 48mm forks (was 46). New shorter Rear shock New redesigned swing arm New head was designed with a slightly lower compression ratio (12.5:1 to 12.3:1), the intake ports are 9% smaller, and the carburetor and ignition settings were also tweaked to provide a smoother and broader powerband.The new head has reduced squish area. De-compression hole in cylinder head eliminated. the 39mm Keihin FCR carburetor gets a double-lipped joint on the intake side so it is less likely to pop off a moveable fuel line joint to accommodate aftermarket tanks better and an additional fuel filter within. New redesigned and beefed up clutch with 12 percent stiffer springs. New clutch lever and perch were created to keep the lever pull nice and manageable. New Renthal aluminum handlebars New kickstands (Aluminum) New Lighter, more durable steel rear sprockets. New front and rear wheel collars New rear brake carriers make removal and replacement of the wheels much easier during trailside flat repairs New front brake lines. Since Honda's patent ran out two years ago, Yamaha was able to more directly route, and thus shorten, the front brake hose. YZ's one piece lower guards. New AIS system added. New muffler. New Revised frame. New Revised mainshaft, 3rd gear. New Relocated crancase breather. New Longer head pipe. 05 models also had the fuel tank redesigned to allow the use of YZ radiator shrouds. All WR's up to '04 had fuel tanks and radiator shrouds unique to the WR. 2006 WR450F Changes Last Year for Steel frame New Digital enduro computer New shift detent pawl assembly. Revised CDI map. Rubber bumper bottoming controll in the forks were replaced with hydraulic lock collars. The cams were revised but the WR still shared the same intake cam as the YZ. 2007 WR450F changes; First year for aluminum frame New ovoid headlight New2.1 gallon YZ-F-style fuel tank New engine and balancer. New cases/head. Crank/trans 2mm wider. Cams, piston, clutch, kickstart shaft, shift detent assy, water pump cover. starter drive carried over from 2006. New integrated oil tank location New camshafts. Revised cam profiles (first year that both cams were unique to WR and not based on the YZ profiles) The new cams have less lift and duration than previous years. New exhaust system New aluminum frame New body styling New LED taillight New quick-access airbox New wave-style disc brake rotors New ProTaper handlebars New wider footpegs New full-coverage skid plate New radiator with bracing New radiator louvers New forged top triple and handlebar clamps. Forks were identical to '06 but triple clamps were new and 2mm wider. New seat cushion designSealed O-ring chain New CDI. Primary ratio to 61:23. New Piston cooling jet added. 2008 WR450F Changes; NONE 2009 WR450F Changes Graphics 2010 WR450F Changes 2010 Model Not Available in US Revised Gripper Seat Cover 2011 WR450F Changes None 2012 WR450F Changes •New fuel injection system delivers quick throttle response and smooth power delivery at nearly all altitudes and temperatures. •Compact YZ250F-based chassis provides a lightweight feeling with plush suspension performance for the most demanding rider. •The new WR450F comes equipped with the latest generation KYB® SSS YZ-type fork and shock, dialed in for off-road use. •New slim bodywork with updated headlight design gives the WR450F an aggressive, tough appearance.