These are 4mm wheel hubs used in many of Tamiya’s RC on-road cars. They replace the standard plastic hub found in stock kits. A slit is provided in the wheel hub, and the gap is tightened with a cap screw. This method of attachment more securely fixes the wheel axle to the hub thereby dropping the 2 x 10 mm stainless steel shaft when removing the wheel is impossible. Furthermore, because it is made of aluminum, you get more accuracy, rigidity, and precision when mounting wheels to your car.
Tamiya wheel hubs are available in 4mm, 5mm and 6mm widths to give you different offset wheel options and tuning possibilities.