MANY DIFFERENT APPLICATIONS ON THE 40 48 50 HP CARBURETORS THAT HAVE A BLACK IDLE CIRCUIT COVER PLATE AT THE TOP OF THE CARBURETORS ONLY AS YOU CAN SEE IN THE PICTURES OF THE LOCATION OF THE ROLLER AND THE CURCUIT COVER PLATE. ONE OF THE ROLLERS GOES ON THE CARBURETOR THROTTLE SHAFT WHICH HAS THE IDLE CURCUIT COVER PLATE WHICH IS BLACK IN COLOR AND ON THE OTHER ROLLER ONE GOES ON THE SPARK ADVANCE LEVER ARM IN THE OTHER PICTURES . ////////////////// THESE ARE BETTER THAN OEM REPLACEMENT PARTS. YOU SHOULD NEVER HAVE TO CHANGE THESE AGAIN UNDER NORMAL OPERATION USE. SO PLEASE MAKE SURE THESE ARE THE CORRECT PARTS FOR YOUR APPLICATION. READ THE DETAILS AND LOOK AT THE PICTURES. THESE  PREVENT FROM BURNING PISTONS DUE TO THE TOO MUCH SPARK ADVANCE AND NOT ENOUGH FUEL CAUSING THE MOTOR TO RUN LEAN ON FUEL . AFTER INSTALLING THIS ROLLER KIT YOU NEED TO RECHECK THE TIMING PICK UP TO MAKE SURE THAT THE TIMING PICK UP IS CORRECT . YOU MAY HAVE TO ADJUST THE CAM ROLLER PICK UP TIMING.I MADE THIS REVISED KIT BECAUSE THESE OEM SLEEVES ALWAYS SPLIT AND CAN CAUSE DAMAGE TO THE MOTOR BY RUNNING LEAN ON FUEL MIXTURE DUE TO TOO MUCH SPARK ADVANCE.I HAVE SPENT A LOT OF TIME COMING UP WITH THIS KIT AND HAVING THESE MFG. .THIS IS NOT A OEM PART IT IS A UPGRADE. THESE ARE BETTER PART THAN THE FACTORY REPLACEMENTS THAT YOU HAVE TO CHANGE MANY TIMES OR THE ONES THAT ARE MADE OF ALUMINUM WHICH WILL WEAR THE CAM LEVER AND THROTTLE CAM OUT.THAT IS THE REASON THE MFG. DID NOT USE THEM ALONG WITH THE CORROSION IT WILL GET ON THE ROLLERS. AGAIN YOU WILL NEVER HAVE TO REPLACE IT UNDER NORMAL CONDITIONS DUE TO THE OEM PART WHICH THE SLEEVE ALWAYS SLITS AND BREAKS.  I HAVE BEEN USING THESE ON MY CUSTOMERS MOTORS FOR OVER 15 YEARS NOW. I WILL SHIP USPS  MAIL