Nylon Spacers for use between your bearings to insure a precision fit. Original parts from Sure-Grip manufactired in the U.S.A. (Size = 5/16" .300). Spacers should always be used to properly lockdown the bearings and create overall wheel and axle strength and stability. Choose Between the 3 types shown (all do the same thing) – 7mm spacing to properly fit between your 2 bearings. (designed for 8mm axle but fits 7mm too). Priced per spacer.