Hex Diameter:12mm / Thickness: 8mm/9mm/10mm/11mm/12mm, multi size, you can choose you want.
Designed for preventing the shaft from falling out and improving the precision for 1:10 RC Crawler. Reduce the gap and closer to the wheel, extend the wheelbase by widening the width of the hex.
Install onto front & rear outer case of the knuckles, this part does not appear in the original and it is sophisticated design for rc crawler car.