Hi, What I have now is a brand new US spec YZ450F -08 with the crappy silencer. I also have a YZ250F -07 that I have put an FMF 4.1 silencer on. I was thinking was getting another 4.1 for the 450, but then I got a crazy idea. Since the 4.1 part number for the 250 and the 450 is the same, I assume the silencer from the 250 would fit(physically) on the 450 no problem. Now that I think about it the bikes might have different size headers since the silencer came with two different collars. I'm not assuming anybody to have experience about this, but do you have any clue whether this would be a waste of my time or worthwhile? Edit: Just to clarify, what I meant is trying the stock 250 exhaust on the 450. The FMF would obviously work but I do not want to switch back and forth since I still intend to ride the 250 as well.