Power Wheelchair and Scooter Caster Bearing The Precision Wheelchair or Scooter Bearing is used on many Wheelchairs as the caster wheel and caster fork stem bearing. Sealed bearings mean maintenance free designReplacement for bearing identification numbers 638-2RS and 638RSInside diameter = 8 mm Outside diameter = 28 mm Width (thickness) = 9 mm Sold individually (each) *Most caster wheels and stem require the purchase of two bearings. Please order quantity 2 for these items. Please message us or call 206 531 0035 for any additional questions.