These replacement o-rings are used with the 500/661 air filter kits.
NOTE: The site will automatically calculate Priority Mail Flat Rate for shipping during checkout. When you only order o-rings (and no other products) we will send them to you via USPS Ground Advantage or First Class (with tracking) when doing so will result in significant savings in the cost of shipping (within the U.S. only - international orders will be sent Priority Mail). We will refund you the difference when that occurs.