This is a hard plastic firewall grommet for 1/0 AWG wire. It has a ip68 rating to help keep water that may splash onto the wire from entering the cab. 

This grommet tightens up as you install it so you can run various size wires through it and still get a great ip68 waterproof rated seal.

These are the highest quality grommets I have ever used.

Fits 1/0 AWG Car audio and welding cable
13-18mm Inner Diameter
IP68 waterproof rating
Sold individually