Installation takes all but 5 mins and from there your legal to ride at ohv parks. It also cuts the sound down a little without taking any power away. Just make sure you put a good thread locker on those bolts when changing out the exhaust cap.
Unfortunately this spark arrestor is only compatible with Big Gun exhaust systems and will not fit your stock WR250F exhaust. The WR250F stock exhaust is already spark arrested unless it has been modified. if the stock spark arrestor has been removed, a replacement can be purchased from Yamaha through our OEM parts fiche here : http://www.motosport.com/dirtbike/oem-parts/YAMAHA/2007/WR250F/EXHAUST . Item # 9 : Part # 5TJ-14780-00-00 SPARK ARRESTER ASSY