The optimal treatment hole is just larger than the rod itself, keeping the rod in close proximity to the hole surface while maintaining easy installation. When using the Genics Cobra Plugs the treatment hole should be 1/16″ smaller than the plug size being used. Refer to the chart below when selecting the best drill size.

Rod/Plug Size

Actual Diameter

Suggested Hole Size

1″ Rod

8.5mm / 1/3″

10mm / 3/8″

2″ Rod

11mm / 7/16″

12mm / 1/2″

3″ Rod

19mm / 3/4″

21mm / 13/16″

4″ Rod

11mm / 7/16″

12mm / 1/2″

9/16″ Plug

14.3mm / 9/16″

12mm / 1/2″

11/16″ Plug

17.5mm / 11/16″

19mm / 3/4″