This thread was lost during the swap, so I'll try to reconstruct it the best I can. 2003; 1st year model. 5 speed trans (YZ had a 4 speed). Electric start. Recall for flywheel nut torque/key shearing issue. 62:22 primary gears. Crankshaft, flywheel, kickstarter shaft, and clutch cover are unique to 2003 model however, '04 and later parts will swap to the '03. An '03 flywheel must be used with an '03 crank. '03 and '04+ cranks and flywheels cannot be mixed. If an '04+ crank/ flywheel is used in an '03, the bolts holding the cam chain guide must be replaced with the '04+ bolts. These bolts are what keep the starter ring gear from contacting the balance shaft. 2004; Replace e-start idle gear w/torque limiter. Eliminate needle bearing/spacer on crank, new crank left half, new flywheel. Special bolt for cam chain damper. Kickstart shaft 20mm (was 18mm), new clutch cover. Revised frame. New shift guide plate. Revise 4th gear. Revised CDI map. New kickstand. Revised crank cases. Revised con rod/piston. Change pimary ratio to 61:23??? 2005; New head casting w/smallerintake port and reduced squish area/less valve shrouding. Add AIS. New triple clamps for 48mm forks (was 46). New Aluminum muffler. Left side panel w/vent. Revised frame. Revised mainshaft, 3rd gear. Relocated crancase breather. Longer head pipe. Revised CDI map. Relocated front brake hose. 1st year of Renthal 7/8 handlebars. NOTE: The new 48mm forks were completely new but were not the AOSS or SSS forks used on the YZ's. The YZ's began using a 48mm fork in 2004 (old style open cartridge design) but the '05+ WR forks were completely different from these also. 2003 was the only year that the YZ and WR shared the same fork. The Titanium footpegs used on the YZ were not used on the WR. The Ti pegs were only used on '05-'06 YZ's and will fit on the WR. 2006; New shift detent pawl assembly. Revised cam profiles. New electronic digital odometer/tripmeter. Revised CDI map. Eliminate rubber bumper in forks and return to lock collar bottom control. Note: In 2007, the YFZ 450 (4 wheeler) had a revised clutch cover that incorporated a piston cooling jet. This revised '07-'09 YFZ clutch cover w/ cooling jet will fit on all '03-'06 WR450's and '03-'05 YZ450's All of the upgrades made up to '06 can be done to all earlier year WR450's. '04-'09 YFZ's (except R and X models) top ends, crank, everything under the clutch cover and magneto cover (including the covers), flywheel and starter drive parts will swap to the WR. The transmission is 16mm wider on the YFZ and none of the tranny gears or shafts will swap. YFZ flywheels will work on '04-'06 WR's but the trigger bar is located in a different position and will retard the ignition timing about 3 degrees. The YFZ uses a 20mm wrist pin on the piston (WR and YZ are 18mm). If a YFZ crank is going to be used in a WR, the YFZ piston must be used as well. The Hot Cranks 471cc YFZ stroker crank will work in the WR is you use a YFZ piston. 2007; Major product change. New aluminum frame. Fork triple clamps are 2mm wider. New cases/head. Crank/trans 2mm wider. Cams, piston, clutch, kickstart shaft, shift detent assy, water pump cover, electric starter drive carried over from 2006. Revised cam profiles unique to WR. New brake rotors. new CDI. Wider footpegs. Primary ratio to 61:23. Engine did away with external oil line and incorporated a piston cooling jet in the right case half. 1st year to use 1-1/8 Pro Taper handle bars. Many parts from the '07+ WR will fit on earlier models. All of the valvetrain components, piston assembly, and most of the clutch parts will swap to earlier bikes. Forls and triple clamps, exhaust header, and wheels will also. The '07-'11 WR transmision, head, clutch cover, clutch, and gear shift assembly will swap directly with '06-'09 YZ '06-'09 YZ Titanium shock spring will fit on all WR's as well as forks, triple clamps, shock, wheels, linkage, fuel tank, and swingarm will swap to '07-'11 WR's WR subframe, seat, and airbox can be modified to fit YZ's. The difference between the two is where the subframe mounts to the frame on the lower left side. YZ ignition, flywheel, or stator will not swap with WR on any year. 2008-2011; No significant changes.