This is a pair of Pro-Line heavy duty wheel extenders with 23mm hex adapters. These hex adapters will allow the HD Velocity 23mm wheels to work on 1/8th scale buggies so they can be converted into monster trucks. These new hexes are much stronger than the stock hexes so they are perfect for monster trucks with big engines (anything above .21 cu in). These hex adapters will fit any 1/8th scale buggy axles.