Exact dimensions, decimal equivalents, tap drill applications, nearest fractional substitutes, and pro tips for every trade.
Size: #29Diameter: 0.1360″ / 3.454 mmSystem: Wire Gauge (Number)Primary Use: #8-32 Tap Drill
If you’ve ever been handed a spec sheet that calls for a #29 drill bit and found yourself staring blankly at your drill index, you’re in good company. Number-gauge drill bits sit in the gap between the common fractional inch sizes most people learned first, yet they’re absolutely essential for precision metalwork, machine shop tasks, tapping threads, and fine woodworking. Understanding exactly what the #29 bit is, where it fits in the numbering system, what it’s used for, and what you can substitute when you don’t have one on hand — that’s the complete picture this guide delivers.
We’ll cover every angle: the exact measurements in both inches and millimeters, the critical tap drill applications, a comparison with neighboring sizes, how to choose between a #29 and its nearest fractional alternatives, and the real-world scenarios where this unassuming little bit makes all the difference.
What Are Number (Wire Gauge) Drill Bits?
Before zooming in on the #29, it helps to understand the system it belongs to. Number drill bits, also called wire gauge drill bits, are part of a standardized American sizing system that runs from #80 (smallest, at 0.0135″) all the way up to #1 (largest, at 0.2280″). Counterintuitively, higher numbers mean smaller diameters — a quirk that trips up many beginners.
This system was developed because fractional-inch drill sizes (1/64″, 1/32″, 1/16″, etc.) jump in relatively large steps. For precision applications — especially in metalworking where thread geometry demands very specific diameters — those steps are too coarse. The number system fills those gaps with finer increments, giving machinists, toolmakers, and engineers the exact hole size they need.
📌 The Number Drill System at a Glance The number drill series contains 80 sizes. Lower numbers (like #1 or #2) are close to 1/4″ in diameter, while higher numbers (like #79 or #80) are thinner than a human hair. The #29 sits in the middle of the practical range, making it one of the most commonly stocked sizes in any machinist’s cabinet.
#29 Drill Bit: Exact Dimensions
The #29 drill bit has a diameter of exactly 0.1360 inches, which converts to 3.454 millimeters. This is a critical specification to commit to memory if you work regularly with #8 machine screws, because #29 is the standard tap drill for the enormously popular #8-32 UNC thread.
#29 Drill Bit0.1360 inches3.454 mmWire Gauge System#8-32 Tap Drill
Size in Context: The Full Measurement Breakdown
| Measurement | Value | Notes |
|---|---|---|
| Drill Number | #29 | Wire gauge / number drill system |
| Diameter (inches) | 0.1360″ | Exact ANSI standard |
| Diameter (mm) | 3.454 mm | 0.1360 × 25.4 |
| Nearest fractional (smaller) | 9/64″ (0.1406″) | +0.0046″ larger |
| Nearest fractional (larger) | 1/8″ (0.1250″) | −0.0110″ smaller |
| Closest letter drill | — (no close letter) | Letter drills start at A = 0.234″ |
| Nearest metric | 3.5 mm (0.1378″) | +0.0018″ larger |
| Shank type (typical) | Straight shank | Fits standard 3-jaw drill chuck |
| Common shank diameter | Matches bit diameter | 3.454 mm / 0.1360″ |
⚠️ Watch the Fractional Trap The nearest common fractional sizes — 1/8″ (0.1250″) and 9/64″ (0.1406″) — are both a significant step away from 0.1360″. For clearance holes or rough woodwork, either might do. For tapping threads, neither will produce the correct thread engagement. Always use a #29 when your spec calls for one.
How Does #29 Compare to Neighboring Sizes?
One of the most useful things you can do with any drill bit spec is look at the sizes immediately above and below it. Knowing whether to go slightly larger or slightly smaller — and by exactly how much — lets you make smart substitution decisions in the field.
| Drill Size | Decimal (inches) | mm | Diff from #29 | Application Notes |
|---|---|---|---|---|
| #27 | 0.1440″ | 3.658 mm | +0.0080″ | Looser clearance; #6-32 body clearance |
| #28 | 0.1405″ | 3.569 mm | +0.0045″ | Just above #29; #8-32 body clearance (sliding) |
| #29 | 0.1360″ | 3.454 mm | — | #8-32 tap drill (standard) |
| #30 | 0.1285″ | 3.264 mm | −0.0075″ | #6-32 tap drill; tighter pilot holes |
| #31 | 0.1200″ | 3.048 mm | −0.0160″ | Smaller pilot work; electronics enclosures |
| 1/8″ | 0.1250″ | 3.175 mm | −0.0110″ | Common fractional — too small for #8-32 tap |
| 9/64″ | 0.1406″ | 3.572 mm | +0.0046″ | Closest fractional — too large for #8-32 tap |
| 3.5 mm | 0.1378″ | 3.500 mm | +0.0018″ | Closest metric — acceptable for some tap work |
As the table shows, the #29 sits in a sweet spot with no close fractional equivalent. The gap between 1/8″ and 9/64″ is 0.0156″ wide — and #29 sits almost exactly in the middle of that gap. This is precisely why number drills exist.
#29 as a Tap Drill: The #8-32 Connection
The single most important application for the #29 drill bit is as the tap drill for the #8-32 UNC (Unified National Coarse) thread. If you’ve spent any time in a machine shop, working on electronics enclosures, HVAC components, automotive parts, or even firearms, you’ve encountered #8-32 screws. They are one of the most ubiquitous machine screw sizes in American manufacturing.
What Is a Tap Drill?
When you want to thread a hole — that is, create internal threads that a machine screw can bite into — you first drill a hole slightly smaller than the screw’s outer diameter, then use a tap to cut the threads into the walls of that hole. The drill you use for that preparatory hole is called the tap drill. Use too large a drill and the threads will be too shallow to hold; use too small a drill and the tap will bind and snap.
For the #8-32 thread, the theoretical tap drill formula gives:
Tap Drill Size = Major Diameter − (1 ÷ Threads Per Inch) For #8-32: Major diameter of #8 screw = 0.1640″
1 ÷ 32 = 0.03125″
0.1640 − 0.03125 = 0.1328″ (theoretical)
Nearest standard size: #29 = 0.1360″ → yields approximately 75–78% thread engagement (ideal for most applications)
The 75% thread engagement target is the industry standard for most materials. Higher engagement (achieved with a smaller tap drill) increases holding strength but also dramatically increases tap breakage risk. The #29 at 0.1360″ hits that sweet spot for steel, aluminum, brass, and most plastics.
Tap Drill Chart: #8 Screw Family
| Thread Size | TPI | Tap Drill | Drill Dia. | Thread % | Material Suitability |
|---|---|---|---|---|---|
| #8-32 UNC | 32 | #29 | 0.1360″ | ~75% | Steel, aluminum, brass, plastic |
| #8-32 UNC (soft) | 32 | #28 | 0.1405″ | ~65% | Aluminum, plastic (easier tapping) |
| #8-32 UNC (hard) | 32 | #30 | 0.1285″ | ~85% | Hard steel (risk of tap breakage ↑) |
| #8-36 UNF | 36 | #29 | 0.1360″ | ~75% | Fine-thread variant, same drill |
| #8 body clearance | — | #19 | 0.1660″ | — | Pass-through hole for #8 screw |
✅ Pro Tip: #29 Also Works for #8-36 UNF The fine-thread variant of the #8 screw — #8-36 UNF — also uses a #29 drill as its standard tap drill. So whether you’re working with coarse or fine #8 machine screws, a single #29 bit covers both tapping scenarios.
Can You Substitute a #29 Drill Bit?
The honest answer: it depends entirely on what you’re doing. For tapping #8-32 threads in metal, substitution is risky. For clearance holes, pilot holes in wood, or general fabrication, the nearest alternatives are often good enough. Here’s the breakdown:
✅ Acceptable Substitutions
3.5 mm metric bit (0.1378″)
Only 0.0018″ oversize. Acceptable for #8-32 tapping in softer materials like aluminum and plastic. Will reduce thread engagement slightly but remain functional.
#28 (0.1405″)
Good for #8-32 tap work in soft materials where you want easier tapping. Also fine for clearance / pilot holes where precision isn’t critical.
9/64″ (0.1406″)
Very close to #28. Will work for non-critical pilot holes, wood joinery, and general fabrication. Too large for ideal #8-32 tapping.
❌ Poor Substitutions
1/8″ (0.1250″)
0.0110″ undersize. Will produce very high thread engagement (90%+), making tapping very difficult in steel and likely to break taps in harder alloys.
#30 (0.1285″)
Designed for #6-32 threads. Using it for #8-32 creates too-shallow a pre-hole, dramatically increasing tap stress. Not recommended.
3.0 mm (0.1181″)
Far too small. Only use for completely different applications — nowhere near suitable for #8-32 work.
Material-by-Material Substitution Guide
| Material | Best Choice | Acceptable Alt. | Reason |
|---|---|---|---|
| Mild steel | #29 (0.1360″) | 3.5 mm (marginal) | 75% engagement ideal; taps break easily here |
| Stainless steel | #29 strictly | None recommended | Hard material — wrong size = broken tap |
| Aluminum | #29 or #28 | 3.5 mm, 9/64″ | Softer metal tolerates slightly larger drill |
| Brass / bronze | #29 | 3.5 mm | Medium hardness; close substitutes OK |
| Plastic / nylon | #28 or #29 | 9/64″, 3.5 mm | Thread-forming (not cutting) often used here |
| Wood (pilot hole) | 1/8″ or 3.5 mm | Any close size | No tapping needed; just preventing split |
| Fiberglass / PCB | #29 | 3.5 mm | Clean hole for standoff hardware |
Real-World Applications of the #29 Drill Bit
Beyond the tap drill role, the #29 shows up in a surprisingly wide range of practical scenarios across multiple trades. Here are the most common contexts where reaching for a #29 is exactly the right move:
1. Electronics and Electrical Enclosures
Sheet metal enclosures for electronics — control panels, junction boxes, server rack panels, electrical cabinets — are almost universally assembled with #8-32 hardware. Panel mounting, standoff attachment, and component fastening all call for tapped #8-32 holes. The #29 is the workhorse bit for every one of those tapping operations. If you do any electrical panel work or enclosure fabrication, keep several #29s on hand because they dull faster than larger bits in sheet steel.
2. HVAC and Ductwork
Sheet metal HVAC fabrication uses #8 screws extensively for joining duct sections, attaching flanges, and mounting dampers. Installers who pre-tap their connection points rather than using self-tapping screws rely on the #29 consistently for field modifications and equipment mounting.
3. Automotive and Motorsports Fabrication
#8-32 screws fasten everything from interior trim panels to sensor mounting brackets in automotive applications. Fabricators building custom brackets, heat shields, and transmission tunnel covers tap holes with a #29 regularly. It’s also common in motorcycle and small engine work where compact fasteners dominate.
4. Firearms and Optics Mounting
The #8-32 and #8-36 threads are standard for many scope base screws, ring screws, and receiver-mounted hardware. Gunsmiths drilling and tapping for scope mounts, rail systems, and sling attachments rely on the #29 as a critical precision tool. In this application, accuracy is paramount — a slightly oversized hole means stripped threads under recoil.
5. Machine Shop and Toolmaking
Jig plates, fixture components, tooling plates, and adjustment mechanisms throughout the machine shop are held together with #8-32 fasteners. The #29 drill is one of the most frequently reached-for sizes in general machine shop work precisely because the #8-32 thread is so prevalent at this scale of work.
6. Woodworking (Pilot Holes)
While the #29’s tap drill role is its claim to fame in metal, woodworkers use it as an oversize pilot hole for #8 wood screws in hardwood — particularly for close-tolerance joinery where splitting risk is high and a snug fit is needed. It’s also used for through-hole pilots when combining wood construction with machine screw hardware.
7. Printed Circuit Board (PCB) Mounting
PCB standoffs — the brass or nylon spacers that keep a circuit board off a chassis — commonly use #4-40 or #6-32 hardware, but #8-32 standoffs appear in larger boards and industrial equipment. Drilling mounting holes in aluminum or steel chassis for these standoffs uses the #29 for tapped-hole installations.
Buying Guide: Choosing the Right #29 Drill Bit
Not all #29 bits are created equal. The material, coating, and geometry of the bit make a real difference in performance and longevity, especially when tapping stainless steel or drilling hardened aluminum repeatedly.
High-Speed Steel (HSS)
HSS is the standard material for number drill bits and the default choice for general use. A quality HSS #29 bit handles steel, aluminum, brass, plastic, and wood without issue. It holds an edge well and can be resharpened. For occasional use or general shop work, a good HSS bit is perfectly adequate.
Cobalt (HSS-Co)
Cobalt bits (typically M35 or M42 grade, containing 5–8% cobalt) offer significantly better heat resistance and hardness than standard HSS. They’re the preferred choice for drilling stainless steel, hardened steels, and titanium. If you regularly tap stainless with #8-32 hardware, a cobalt #29 bit will last dramatically longer than standard HSS.
Black Oxide Coating
Black oxide is a mild surface treatment that reduces friction slightly and provides minimal corrosion resistance. It’s common on budget HSS sets and adds modest value over bare steel. Fine for general use.
TiN (Titanium Nitride) Coating
TiN-coated bits have a distinctive gold color and offer meaningfully improved hardness and reduced friction. They stay sharper longer in hard materials and are a solid upgrade for shop use. Note that once the coating wears through the cutting edge, the advantage disappears — these bits cannot be resharpened without losing the coating.
Solid Carbide
Carbide #29 bits are used in CNC machining and production environments where cycle times are critical and bit life must be maximized. They are brittle compared to HSS (don’t drop them), require rigid machine setups, and are overkill for hand-drill or drill press work. Not a typical shop-floor choice.
💡 Buying Recommendation For most workshops: HSS black oxide or TiN-coated #29 from a reputable brand (Irwin, Chicago Latrobe, Norseman, Drill America, Greenfield). Buy in packs of 2–3 because #29s are frequently used for tapping and dulling one mid-project is a real problem. Expect to pay $3–8 for quality individual bits, or look for number drill sets (#1–#60 or #1–#80) that include #29 along with all other common sizes.
Tips for Using the #29 Drill Bit Effectively
Use Cutting Fluid in Metal
Small-diameter bits generate enormous heat relative to their mass. Any time you’re drilling steel, stainless, or brass with a #29, use a cutting fluid — even a drop of machine oil or cutting wax. This dramatically extends bit life and produces a cleaner hole that taps more smoothly.
Peck Drilling in Deep Holes
For holes deeper than about 3× the drill diameter (roughly 3/8″ for a #29), use a peck drilling technique — drill a short distance, retract to clear chips, drill more, retract again. This prevents chip packing in the flutes, which causes heat spikes and bit breakage.
Center Punch Before Drilling
At 0.136″, a #29 bit will wander on smooth metal without a center punch mark. Always use a punch to create a divot for the bit to seat in. In CNC work this is handled by the tool path, but for drill press and hand work it’s non-negotiable for accuracy.
Drill Speed
Smaller bits need higher RPM for efficient cutting but generate more heat. General guidance for a #29 in common materials:
| Material | Recommended RPM | Notes |
|---|---|---|
| Mild steel | 1,500 – 2,500 | Use cutting oil |
| Stainless steel | 800 – 1,200 | Slow, steady; cobalt bit preferred |
| Aluminum | 3,000 – 5,000 | Fast, light feed; cutting fluid helps |
| Brass / bronze | 2,000 – 3,500 | No cutting fluid needed |
| Plastic | 1,500 – 3,000 | Light pressure to avoid melting |
| Hardwood | 1,200 – 2,500 | Sharp bit; go slow in end grain |
Deburr the Hole Before Tapping
After drilling with your #29, use a larger drill bit (held by hand), a countersink, or a deburring tool to lightly chamfer the hole entry. This removes the sharp burr that drilling leaves behind, prevents the tap from cross-threading, and gives the first few threads of the screw a clean lead-in.
Don’t Force a Dull Bit
A dull #29 feels like it’s not cutting — you add more feed pressure, generate more heat, and increase the chance of work-hardening the material (especially stainless) and snapping the bit. When cutting starts feeling sluggish or squeaky, replace or resharpen the bit before continuing.
Number Drill Sizes: #25 to #35 Reference Chart
To put #29 in full context, here’s the surrounding neighborhood in the number drill system — the sizes you’ll most commonly pull alongside it in precision metalwork:
| Drill # | Decimal (in) | mm | Nearest Fractional | Primary Tap Use |
|---|---|---|---|---|
| #25 | 0.1495″ | 3.797 | 5/32″ (0.1563) | #10-24 UNC tap drill |
| #26 | 0.1470″ | 3.734 | — | Clearance / misc. |
| #27 | 0.1440″ | 3.658 | — | #6-32 body clearance |
| #28 | 0.1405″ | 3.569 | 9/64″ (0.1406) | #8-32 alt (soft material) |
| #29 | 0.1360″ | 3.454 | — | #8-32 UNC & #8-36 UNF |
| #30 | 0.1285″ | 3.264 | — | #6-32 UNC tap drill |
| #31 | 0.1200″ | 3.048 | — | Misc. pilot / clearance |
| #32 | 0.1160″ | 2.946 | — | — |
| #33 | 0.1130″ | 2.870 | 7/64″ (0.1094) | #5-40 UNC tap drill |
| #34 | 0.1110″ | 2.819 | — | — |
| #35 | 0.1100″ | 2.794 | — | — |
Frequently Asked Questions About the #29 Drill Bit
What size is a #29 drill bit in inches?
A #29 drill bit is 0.1360 inches in diameter. This is its exact ANSI standard measurement and is the figure you should use when specifying hole tolerances or checking your drill with a caliper. What is a #29 drill bit in mm?
The #29 drill bit is 3.454 millimeters in diameter (0.1360 × 25.4 = 3.4544 mm). The nearest standard metric drill size is 3.5 mm, which is 0.046 mm oversize — acceptable for some applications but not ideal for precision tapping. What is the #29 drill bit used for?
The #29 drill bit’s primary application is as the tap drill for #8-32 UNC machine screw threads. It is also the tap drill for #8-36 UNF threads. Beyond tapping, it is used for clearance holes, pilot holes, and any application where a hole diameter between 1/8″ and 9/64″ is required. What drill bit is closest to a #29?
The closest sizes to a #29 (0.1360″) are: #28 at 0.1405″ (+0.0045″) and #30 at 0.1285″ (−0.0075″). In metric, 3.5 mm (0.1378″) is the closest available standard size, sitting just 0.0018″ over the #29. In fractional inches, the nearest are 1/8″ (0.1250″) and 9/64″ (0.1406″) — both are a meaningful step away. Can I use a 3.5 mm bit instead of a #29 for tapping #8-32 threads?
In soft materials like aluminum and plastic, a 3.5 mm bit (0.1378″) is an acceptable substitute for a #29 when tapping #8-32. The 0.0018″ oversize reduces thread engagement slightly (from ~75% to ~70%) but remains functional. In steel and stainless steel, use a true #29 for proper thread engagement and to reduce tap breakage risk. What’s the difference between #29 and #30 drill bits?
The #29 (0.1360″) is 0.0075″ larger than the #30 (0.1285″). The #30 is the standard tap drill for #6-32 UNC threads, while the #29 handles #8-32 UNC. Using a #30 instead of a #29 for #8-32 tapping would create very high thread engagement (approximately 90%+), which greatly increases the risk of a broken tap, especially in steel. How do I know if I have a #29 drill bit if the marking has worn off?
Use a digital caliper to measure the shank or the fluted section of the bit. A #29 will measure 0.135–0.137″ (approximately 3.43–3.48 mm). If you don’t have a caliper, a drill gauge plate (a metal plate with precisely sized holes) can identify the size by which hole the bit just fits through. Is a #29 drill bit the same as a letter drill?
No. Number drill bits (#1–#80) and letter drill bits (A–Z) are two different subsystems within the broader wire gauge drill sizing family. Letter drills are all larger than number drills — the smallest letter drill is A at 0.234″, while the #29 is only 0.1360″. They occupy different size ranges and serve different applications.
Conclusion
The #29 drill bit is one of those sizes that earns its place in any serious workshop through relentless usefulness. At 0.1360 inches (3.454 mm), it occupies a gap in the sizing spectrum that no fractional bit fills, and its role as the standard tap drill for #8-32 UNC threads makes it indispensable across metalworking, electronics, HVAC, automotive, and firearms fabrication.
Understanding its exact dimensions, how it compares to neighboring sizes, where substitutions work and where they don’t, and how to use it correctly in various materials will save you from the twin frustrations of stripped threads and broken taps. Keep at least two sharp #29 bits in your index at all times — it’s a small investment that pays off every time you reach for it.
For the best results, use a genuine HSS or cobalt #29 from a reputable manufacturer, pair it with cutting fluid in metal, center-punch before drilling, and deburr before tapping. Do those four things and the #29 will reward you with clean, accurate holes and perfectly formed threads every single time.
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