When determining which size wheelchair bearing or scooter bearing to purchase, measure both the inside and outside diameter of the existing bearing, and write down the bearing reference number (if available). You can also measure the diameter of the wheelchair axle or caster stem bolt to find out what the inside diimension of the bearing is. Never purchase based on the image. If you need help give us a call at (800) 983-3602, or drop us a line at support@enableyourlife.com.