I would use a 5/8 drill bit which is almost exactly 16mm.
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For a cut tap the correct drill size is a #17 drill, which has a diameter of .173 inches.
The recommended drill size for a 12 screw is 5/32 inches.
A 6mm drill bit can drill a hole that is approximately 15/64 inches in size.
For a 6 screw, you should use a drill bit that is 7/64 inches in size.
A drill bit size of .376 inches is equivalent to a 3/8 inch drill bit, which is slightly smaller than 3/8 inch (0.375 inches) and larger than 13/32 inch (0.40625 inches). In metric terms, this is approximately 9.55 mm. It’s important to choose the right drill bit size based on the specific requirements of your project.
size 6 rings start at 10 then just add the size to it,lets say its a size 6 so,10+6=16mm
A 16mm diameter is roughly equivalent to about 0.63 inches, which is slightly larger than a U.S. nickel coin. To visualize it, think of the size of a small button or a large marble. It's a common size in various applications, such as in jewelry or mechanical components.
The recommended 5 drill bit size for drilling holes in metal is 0.2055 inches in diameter.
The recommended drill bit size for a 1/4 lag screw is 3/16 inches.
The recommended 10 drill bit size for drilling holes in metal is 3/16 inches.
The recommended 6 drill bit size for drilling holes in metal is 7/64 inches.