This MultiCart/Rock N Roller Wheel is a replacement 8" wheel (not a caster) for the R6 or R10 model cart. It is an 8 x 2.5" 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.