Yellow, zinc-passivated steel double worm screw head attaches to drain rods for breaking up and drawing out obstructions in drains. Compatible with Silverline Expert Drain Rod Set and most third party sets. 16mm bore. Yellow, zinc-passivated steel double worm screw head attaches to drain rods for breaking up and drawing out obstructions in drains. Compatible with Silverline Expert Drain Rod Set and most third party sets. 16mm bore.