I have searched on the year to year differences on the WR450, and I have not found much engine wise. I know of the starter key shearing issue in '03 that was resovled in '04. Also I think the head casting may have changed at one point to get rid of the manual decompression hook up. Aside from these small changes, has there been anything that would cause major interchangability issues throughout the years? I know that the YZ's have gone under changes speciffically with the transmission that doesn't make parts interchangable. Thanks in advance for the help.