For coarse threads (13 threads per inch) use a 27/64 drill
.50 drill bit = 8mm rubber grommet
The recommended drill bit size for a 15/64 inch hole is a 15/64 inch drill bit.
The recommended size of the drill bit for a 7/16 inch hole is a 7/16 inch drill bit.
The recommended drill bit size for a 5/16 inch screw is 1/8 inch.
A 3/16 drill bit.
The 'R' drill is 0.339 inch, which converts to just a tiny bit less than 11/32 of an inch
An 'O' drill bit is a decimal size, 0.3160 inch, which is between 5/16 and 21/64 on the fractional inch scale.
The recommended drill bit size for a 10 screw is 3/16 inch.
The recommended drill bit size for a 6 screw is 7/64 inch.
It's 0.1160 of an inch, or a tiny,tiny bit less than 1/8 inch.
A 4 mm drill bit is equivalent to 5/32 inch.
That size is correctly 0.375, and is equal to 3/8 of an inch.