This MultiCart/Rock N Roller Wheel is a replacement 10" wheel (not a caster) for the R10 or R12 model cart. It is a 10 x 3.0" pneumatic wheel with a 20mm bore that uses a 1.5 x 1/8" cotter key to fasten the axle. It is designed specifically for carts manufactured after 2001 with 20mm tube axles.