1/2-13 Tap Drill Size and Correct Drill Bit
Use a 27/64 drill bit
1/2-13 · 0.4219 in / 10.72 mm · ~75% thread
1/2-13 is a larger coarse UNC thread used in machinery and structural work. A 27/64 inch bit is the standard tap drill.
At a glance
| Tap drill | 27/64 |
|---|---|
| Inches | 0.4219 in |
| Millimetres | 10.72 mm |
| Major diameter | 0.5 in |
| Pitch | 0.0769 in |
| Threads per inch | 13 |
Why this size
drillØ = major − (pitch × %thread) = 0.5 − (0.0769 × 0.75) = 0.4423 in → next standard drill 27/64 (0.4219 in / 10.72 mm)
This is a larger thread so accurate sizing matters. For a clearance hole a 17/32 bit (0.5313 in) is the close fit — bigger than the tap drill, so the bolt passes through unthreaded.
Clearance holes for 1/2-13
A clearance hole lets the bolt pass through unthreaded – close fit keeps parts aligned, free fit allows movement.
| Tap drill (cuts threads) | 27/64 |
|---|---|
| Close fit clearance | 17/32 bit |
| Free fit clearance | 39/64 bit |
Related sizes
Compare with adjacent threads – coarse, fine and the opposite measurement system: