FMF Powercore 4 Muffler Aluminum for Yamaha YZ/WR 400 426 044013 1998-2002 THE LATEST VERSION OF THE POWERCORE 4 DESIGN INCORPORATES A HI-FLOW CORE WHICH HAS MORE SURFACE AREA TO ABSORB MORE SOUND WHILE FLOWING MORE EXHAUST CONSTRUCTED WITH AN EXTRUDED ALUMINUM CANISTER, A STAINLESS STEEL MIDPIPE THAT IS MIG/TIG WELDED AND HAS SPACE AGE INTERNAL PACKING LIGHTER, STRONGER AND MORE POWERFUL THAN STOCK EXHAUSTS POWERCORE 4 S/A HAS A U.S. FORESTRY APPROVED MAINTENANCE FREE, STAINLESS STEEL SPARK ARRESTOR SCREEN