I have done a lot of searching but I can't find any thing specific to my issue. I purchased an 05 yz450f pipe on ebay for my 03 wr450f however the diameter of the mid pipe is slightly smaller than my header. I'm running the canadian wr so if I don't if that would make a difference. The header will slide into the mid pipe with out the gasket but not with the gasket that came with the pipe. And not that I could remove and reuse my wr gasket but that would not fit into the mid pipe with the header as well. Its pretty obvious that the mid pipe on the the stock wr pipe is larger than the pipe I have purchased. The pipe fits up to the bike as it should and they will slide together, just not with the gasket. I'm going to bring both the header and pipe to the dealer today and see what they say and if simply a new gasket will fix the problem. Worst case scenario I can buy the new gasket and if it is larger I can always bring the pipe to a muffler shop and have them expand the end of the pipe to accept the gasket and header. I could also cut the end off my wr pipe and weld it to the new pipe but I think I would send it back to the seller before that. I just hope I haven't been sold a 250 pipe as a 450!!!!!!!!!!