letter drill · nominal diameter
O
drill bit size.
A O bit measures 0.3160 in across (8.026 mm).
The inch decimal is the published nominal designation; millimetres are a conversion.
Calibrated relative diameter · 1 in reference
O · letter
0.3160 in
8.026 mm DesignationOletter
Inch decimal0.3160nominal
Metric conversion8.026millimetres
Where this size sits from 0 to 1 inch O · 8.026 mm on a zero to 25.4 millimetre scale. Neighbor marks: 8 mm, 8.1 mm.
Measured neighbors
What sits beside O.
“Nearest” means nominal diameter only. It does not make a neighboring size acceptable for a tolerance, tapping, reaming, or fastener application.
Source record
ASME.
The standard text is paywalled. Number and letter series are cross-checked against public manufacturer charts; exact standard edition/page verification remains a release gate.
Open source record