Yes my friend just bought a 06 YFZ 450 and i have one as well... Well we were going over his bike to make sure everything looked ok and when we opened up the air box we noticed there wasnt an air filter in the air box... and we noticed that there wasnt the ice cream cone shaped air filter holder that comes on the bike stock, that holds the air filter in place.. but what we did notice was that it looked like that was removed and some sort of aftermarket bracket was installed in it... there was about a 1in bracket that stuck out and it has an outside diameter of 4-1/2 in.. Now is this for a clamp on style bracket for a clamp on filter than has to have a 4-1/2 OD to slip around the bracket or what.... it also looked like it has very fine threads that go all the way around as well.. so i dont know if a filter slips around this and gets clamped on or it actually gets screwed on there... PLEASE HELP ME.. if you click on this link and you look at the far left of the 3 brackets thats exactly what is sticking out into my air box and so is that for a filter to slip over

http://velocityfilters.com/_mgxroot/page_10767.html

 

he might have gotten the 250r air box, it's a common mod with air boxes. hopefully that helps?

 

hey thanks for the info... But if it is the 250R air mod is that a bad thing??? And correct me if im wrong but would the airfilter have to slide over the 4-1/2 in Outside diameter bracket??? B/c thats a pretty large filter if thats the case or is that about normal?? And i need to know where he can get a filter for his bike b/c he doesnt have one that will fit that.... And what ever was installed is clearly aftermarket bc you can see where the rivets were drilled out from the stock part that was thre and the new 4-1/2 diameter was installed with new allen bolts... it was done very well just need to know what kind of filter to get or what size??

 

look at the link i posted in my post and take a look

 

try calling velocity? if it is there's i guess they could tell you...is it possible to ask the guy you bought it from? i dunno, sorry man