1-1/4 Tap Drill Size and Correct Drill Bit
Use a 1-1/8 drill bit
1-1/4-7 · 1.1250 in / 28.58 mm · ~75% thread
1-1/4-7 is a large coarse UNC thread for heavy machinery, dies and structural work. A 1-1/8 inch bit is the standard tap drill at 75% engagement.
At a glance
| Tap drill | 1-1/8 |
|---|---|
| Inches | 1.1250 in |
| Millimetres | 28.58 mm |
| Major diameter | 1.2500 in |
| Pitch | 0.1429 in |
| Threads per inch | 7 |
Why this size
1-1/4-7 tap drill: drillØ = major − (pitch × 0.75) = 1.2500 − (0.1429 × 0.75) = 1.1428 in → nearest standard bit = 1-1/8 (1.1250 in)
At this scale the tap pulls hard: drill deep enough, use a spiral-point (gun) tap, and cut with cutting oil. A 1-3/16 in bit (1.1875 in) gives a looser ~60% thread for hard-chip materials.
Related sizes
Nearby sizes
The large coarse UNC threads around this size: