Tap Drill Chart & Thread Reference
Complete tap drill size reference for UNC, UNF, and metric threads. Includes clearance hole sizes, Helicoil drill charts, torque specifications, and thread engagement calculator.
Unified National Coarse (UNC)
Standard thread for general purpose applications. Coarse threads are stronger, faster to assemble, and more forgiving of damaged threads. Use UNC unless you have specific reasons for fine thread.
| Thread Size | Major Dia | TPI | Tap Drill | Tap Drill (dec) | Close Fit | Free Fit |
|---|---|---|---|---|---|---|
| #2-56 | 0.0860" | 56 | #50 | 0.0700" | #43 | #41 |
| #4-40 | 0.1120" | 40 | #43 | 0.0890" | #32 | #30 |
| #6-32 | 0.1380" | 32 | #36 | 0.1065" | #25 | #18 |
| #8-32 | 0.1640" | 32 | #29 | 0.1360" | #16 | #9 |
| #10-24 | 0.1900" | 24 | #25 | 0.1495" | #7 | #2 |
| #10-32 | 0.1900" | 32 | #21 | 0.1590" | #7 | #2 |
| 1/4-20 | 0.2500" | 20 | #7 | 0.2010" | F | H |
| 5/16-18 | 0.3125" | 18 | F | 0.2570" | P | Q |
| 3/8-16 | 0.3750" | 16 | 5/16 | 0.3125" | W | X |
| 7/16-14 | 0.4375" | 14 | U | 0.3680" | 29/64 | 15/32 |
| 1/2-13 | 0.5000" | 13 | 27/64 | 0.4219" | 33/64 | 17/32 |
| 5/8-11 | 0.6250" | 11 | 17/32 | 0.5312" | 41/64 | 21/32 |
| 3/4-10 | 0.7500" | 10 | 21/32 | 0.6562" | 49/64 | 25/32 |
| 7/8-9 | 0.8750" | 9 | 49/64 | 0.7656" | 57/64 | 29/32 |
| 1-8 | 1.0000" | 8 | 7/8 | 0.8750" | 1-1/64 | 1-1/32 |
Thread Fundamentals
Understanding Thread Specifications
Thread specifications define the geometry of screw threads. Understanding these specs is essential for proper fastener selection and tapped hole design.
Format: Size-TPI (e.g., 1/4-20)
- Size: Major diameter (fraction or number)
- TPI: Threads per inch
- UNC: Unified National Coarse
- UNF: Unified National Fine
Format: M[dia]x[pitch] (e.g., M8x1.25)
- M: Metric designation
- dia: Major diameter in mm
- pitch: Distance between threads in mm
- Coarse pitch is implied if not specified
Tap Drill Selection
The tap drill creates the hole that will be threaded. The drill size determines thread engagement—typically 75% for standard applications. Higher engagement (80-85%) is used for softer materials or high-stress applications.
For 75% thread engagement:
Tap Drill = Major Diameter - (0.75 Ă— Pitch)For imperial: Pitch = 1/TPI. For metric: use pitch directly in mm.
Coarse vs Fine Threads
Choose thread type based on application:
| Factor | Coarse (UNC) | Fine (UNF) |
|---|---|---|
| Assembly Speed | Faster | Slower |
| Tensile Strength | Good | Higher |
| Strip Resistance | Better in soft materials | Better in hard materials |
| Vibration Resistance | Standard | Better |
| Adjustment Precision | Standard | Finer |
| Damage Tolerance | More forgiving | More sensitive |
| Common Applications | General purpose, cast iron, aluminum | Aerospace, automotive, thin walls |
Clearance Hole Selection
Clearance holes allow bolts to pass through without engaging threads. Two standard fits:
- Close Fit: Minimal clearance for precise alignment. Use when hole position accuracy is critical.
- Free Fit: More clearance for easy assembly. Use for standard applications and when slight misalignment is expected.
Thread Inserts (Helicoil)
Wire thread inserts provide durable, wear-resistant threads in soft materials or repair stripped threads. The process:
- Drill the oversized hole (larger than standard tap drill)
- Tap with STI (Screw Thread Insert) tap
- Install the coiled insert using insertion tool
- Break off the driving tang (if applicable)
The result is a thread that can be stronger than the original, with better wear resistance and the ability to use steel fasteners in aluminum bodies without galling.
Torque Specifications
Proper torque ensures reliable joint clamping without damaging threads. Key factors:
- Fastener grade: Higher grades allow higher torque. Grade 8/Class 10.9 is typically 40% stronger than Grade 5/Class 8.8.
- Thread lubrication: Dry threads require 25-30% less torque than oiled threads to achieve the same clamping force.
- Material: Stainless steel requires 50% less torque due to galling tendency.
- Coatings: Plated fasteners (zinc, cadmium) typically require 10-15% less torque.
Stainless steel fasteners are highly prone to galling (cold welding). Always use anti-seize compound or lubrication. Never exceed 50% of carbon steel torque values. If resistance is felt during tightening, stop immediately—the fastener may be galling.
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Frequently Asked Questions
What size tap drill for 1/4-20?
The standard tap drill for 1/4-20 UNC is a #7 drill (0.201"). This provides approximately 75% thread engagement. For clearance holes, use an F drill (0.257") for close fit or an H drill (0.266") for free fit.
What is the difference between UNC and UNF?
UNC (Unified National Coarse) has fewer threads per inch, making it faster to assemble and more tolerant of damaged threads. UNF (Unified National Fine) has more threads per inch, providing higher tensile strength, better vibration resistance, and finer adjustment capability. Use UNC for general purpose; UNF for aerospace, automotive, and precision applications.
What drill size for Helicoil inserts?
Helicoil drills are larger than standard tap drills to accommodate the insert. For example, a 1/4-20 Helicoil requires a 17/64" (0.2656") drill instead of the standard #7 (0.201"). Always use the STI (Screw Thread Insert) tap, not a standard tap. The insert then creates standard 1/4-20 threads.
How deep should tapped holes be?
Minimum thread engagement is typically 1.5x the bolt diameter for steel into steel, 2x for steel into aluminum, and 2.5x for steel into plastic. For blind holes, add 2-3 thread pitches beyond the thread depth for tap runout and chip clearance. For example, a 1/4-20 bolt in steel needs at least 0.375" of thread, plus about 0.15" for clearance.
What is the torque for a 3/8-16 Grade 5 bolt?
A 3/8-16 Grade 5 bolt with lightly oiled threads should be torqued to approximately 31 N·m (23 ft·lbs). For Grade 8, increase to 44 N·m (33 ft·lbs). Reduce these values by 25-30% for dry threads, 50% for stainless steel, and 15-20% for lock nuts.