4 Gauge AWG Metal Firewall Grommet Bushing Audio Amplifier Wire/Cable Install High Quality Nickel Plated Brass, Adds a Durable and Clean touch to your install. Compresses around your 4 Gauge Cable to lock it in place, and seal out Moisture and Air Leaks. Can be mounted to Metal, Wood, Fiberglass, or even Plastic. (Such as Firewalls, Floor Pans, or even Speaker Enclosures.) Extra Long Mounting Threads compared to Competitors. (Can be Mounted to surfaces up to 11/16's 17.5mm thick) Required Hole Size: 7/8" (23mm) (Use a Hole Saw, Step Drill, or Knockout Punch)