ok thanks...Someone said they tried mounting the muffler and it didn't work so I was going by what he said. So many different answers... The muffler is a "slip-on" which means it fits the stock header. It does fit the stock header which just happens to be the same diameter as the GYT-R header. Funny that, yamaha using optomised tubing sizes. My procircuit muffler from the Ti GP system just about fits on the stock header. The stock header is just fine. The '06 WR is no different than the '03. Only '06 YZ's different to '03 - '05 450's.