OK...here is the scoop: I just rebuilt a wr426. I had to buy a new head because the valve dropped and chewed the old one back up. I had the decompression shaft that slides into the head you hook to the decompression cable. And I swear..that thing just doesn't fit in this head. Everything else fits fine...I mean everything. I even plugged the hole and started it and it runs good. SOOO...why doesn't that piece fit? I ordered a whole new assy from Yamaha and that is the same as the one I had, and it doesn't fit at all. It appears the diameter of the butterfly and shaft are just to wide. Could this be a head for a 450? Plz help, I am really wanting to ride this bike bad, I have put hrs and hrs of time getting this thing together, and this is the very last thing needing done!!! Thx a lot.