The correct size O-Rings for the K1100 center stand sleeves are 20mm x 14mm x 3mm. These are often used as 'prop savers' for electric RC planes and may be available at your local hobby shop. If not, they are available on ebay for $0.99 USD for a 20-count package. I ordered them on ebay and after taking a while to come on a slow boat from China they fit fine.