I've gone back to lighter plastic hexes and stopped using these due to the grub screws rounding out. You could replace the grub screws with higher quality ones that don't round out, or buy clamping hexes that use a step screw instead for a better result.
The plastic one keep getting stuck in the rims. This is tough in between heats. So I just bought these now. I use 5mm in the front of the car and 6 mm in the back. Some times on Tamiya cars the 30mm sponge in the back can touch.