The hole size for the round terminal is 8mm. These are called static dischargers or electron dischargers. They discharge static electricity that has built up on a car's body into the air. It is said to contribute to improved fuel efficiency and quieter operation. They are used by attaching them to bolts. It is said that installation on the negative terminal of the battery or at ground points is effective. It's a hot topic on [Minkara], Japan's largest car and automobile SNS, and many people have installed and reported on them. I had the tools to make those static dischargers, so I tried DIY. Please understand that this is an original DIY product before purchasing. Also, please install at your own risk. Regarding the terminal size, combinations of M6, M8, and M10 are possible, so please consult. #StaticElectricityRemoval #Quietness #Earthing #FuelEfficiencyImprovement #StaticDischarger #TorqueUp #StaticElimination #ElectronDischarger #StaticElectricityMeasures #DischargeCable