1/4-20 Tap Drill Size: Three 5.2 mm Carbide Drills Compared
The standard tap drill for a 1/4-20 UNC thread is a #7 drill (0.201"), which gives about 75% thread. In CNC production a slightly larger carbide drill, 0.204" or 5.2 mm, is the usual choice: it leaves about 70% thread, which is still full strength for a 1/4-20, and the tap runs with less torque and breaks less often. We carry three carbide 1/4-20 tap drills of this size. They look almost identical on paper, so here is what actually differs.
The three drills side by side
| UP TOOL D2HC-204 | EASYCUT ED217 | EASYCUT ED216 | |
|---|---|---|---|
| Cutting diameter | 0.204" (5.18 mm) | 5.2 mm (0.205") | 5.2 mm (0.205") |
| Diameter tolerance | — | m7: +0.004 / +0.016 mm | h7: 0 / −0.012 mm |
| Shank | 1/4" (6.35 mm) | 6 mm, DIN 6535 HA | 6 mm, DIN 6535 HA |
| Length of cut / overall length | 1.4" / 3.25" | 35 mm (1.38") / 82 mm (3.23") | 35 mm (1.38") / 82 mm (3.23") |
| Geometry | 2 flutes, 140° point, 5xD, through-tool coolant — the same on all three | ||
| Coating | HR coating | PVD coated | PVD coated |
| Materials (ISO groups) | P, M, K, N | P, M, K, N, S | P, M, K, N, S, H |
| Unit system | Inch | Metric | Metric |
How to choose
1. Shank: match it to your holder
This is the difference that matters most. The UP TOOL drill has a 1/4" shank, both EASYCUT drills have a 6 mm shank. In an ER collet either one clamps fine. In a shrink-fit or hydraulic chuck, or in a 1/4" end mill holder, the shank has to match the bore exactly — 6 mm is 0.236", about 0.014" smaller than 1/4". If your holders are inch, take the D2HC-204. If they are metric, take one of the EASYCUT drills.
2. Tolerance: h7 or m7
Both EASYCUT drills are 5.2 mm nominal, but they are ground to different tolerances. The ED216 (h7) is never larger than 5.200 mm. The ED217 (m7) is never smaller than 5.204 mm, which is typical for drills made specifically for tapping holes: the hole comes out a few microns larger and the tap runs a little easier. For a 1/4-20 thread the difference is under 1% of thread, and both sizes stay well inside the 2B minor diameter limit (0.196–0.207"). In practice either works; pick the ED217 if your taps are close to the limit on torque, the ED216 if you also use the drill for holes that need to stay on or under size.
3. Material
All three drill steel, stainless steel, cast iron and aluminum. The EASYCUT drills are also listed for heat-resistant alloys (S), and the ED216 for hardened materials (H) as well. For a shop that mostly cuts steel and aluminum, this will not decide the choice.
1/4-20 UNC tap drill sizes and thread percentage
| Drill | Diameter, in | Diameter, mm | Thread, approx. |
|---|---|---|---|
| 5.1 mm | 0.2008 | 5.10 | 76% |
| #7 | 0.2010 | 5.11 | 75% |
| 13/64" | 0.2031 | 5.16 | 72% |
| UP TOOL D2HC-204 | 0.2040 | 5.18 | 71% |
| EASYCUT ED216 / ED217 | 0.2047 | 5.20 | 70% |
Thread percentage is calculated as 76.98 × TPI × (major diameter − drill diameter). Around 70–75% is the normal range for cut taps in steel; going higher adds very little strength and a lot of tap torque. For form taps the hole is larger — about 5.5–5.6 mm (0.217–0.220") for 1/4-20 — so these drills are not the right size for forming.
FAQ
All three drills are in our carbide drills catalog and can be ordered online 24/7. For coolant choice when tapping aluminum, see our tapping torque test.