15/64 Drill Bit Equivalent: Your Ultimate Guide to Getting the Perfect Fit
You’re deep into a project, following instructions to the letter. Everything is going smoothly until you hit a frustrating snag: the directions call for a 15/64 drill bit, a size that seems to have vanished from your drill case. This scenario is incredibly common, as the 15/64″ bit is a specialty size often omitted from standard, everyday drill bit sets.
This missing tool can bring your entire project to a screeching halt. But before you abandon your work to make a trip to the hardware store, understand that a precise equivalent is likely already in your toolbox. This guide will help you identify the perfect substitute and understand why choosing the right one is critical for your project’s success.
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Understanding the 15/64″ Drill Bit: Size and Purpose
To find a proper equivalent, you first need to know the exact dimensions of the target bit. The 15/64″ drill bit has a very specific diameter that serves critical functions in both woodworking and metalworking.
In decimal form, 15 divided by 64 equals 0.234375 inches. When converted to metric, this is approximately 5.95 millimeters. This precise size is most famously known as the designated tap drill size for creating 75% thread engagement for a 5/16″-18 UNC screw thread, a very common fastener size in machinery and automotive applications.
Using the wrong size bit for tapping can have disastrous consequences. A bit that is too large will create shallow, weak threads that can easily strip under pressure. A bit that is too small will make it nearly impossible to turn the tap, risking breakage of the tap itself within the workpiece—a costly and difficult problem to fix.
The Critical Importance of a Precise Equivalent
The search for an equivalent is more than just finding a “close enough” option. The tolerance required by your project dictates how much variance is acceptable. A clearance hole for a bolt in a piece of wood can be slightly oversized without issue, but a hole for a precision-fit dowel pin or a tapped thread in steel demands accuracy down to a few thousandths of an inch.
Choosing an ill-fitting substitute can lead to a host of problems. A slightly oversized hole can result in loose joints, wobbly fasteners, and a general lack of structural integrity. Conversely, an undersized hole can cause wood to split, screws to bind and strip, and metal to deform under the stress of forcing a fastener.
The Best 15/64 Drill Bit Equivalents: Metric, Number, and Letter Sizes
Your drill bit collection likely contains bits from different sizing systems: fractional inch, metric, and the less common number and letter gauge sizes. The perfect equivalent for the 15/64″ bit is often found in these other systems. Here are your best options, ranked from closest fit to more situational choices.
The Near-Perfect Match: The Letter ‘A’ Drill Bit
For those with a more comprehensive drill index, the letter ‘A’ drill bit is the undisputed champion of 15/64″ equivalents. Its diameter is 0.2340 inches, making it only 0.0004 inches smaller than the 15/64″ bit. This difference is so negligible that for virtually all applications, including precise tapping, it is a perfect substitute.
The incredibly small variance means you get the best of both worlds. For tapping, it provides just enough material for the tap to create strong, well-formed threads. For clearance or locating pins, it provides a snug, precise fit with minimal play.
The Metric Contenders: 5.9mm and 6mm
The metric system offers two excellent candidates, each with its own ideal use case. The closest metric size is the 5.9mm drill bit. It measures 0.2323 inches in diameter, making it about 0.0021 inches smaller than the 15/64″ bit. This slightly undersized nature makes it a superb choice for tapping threads, as it ensures maximum thread engagement and strength.
On the other side is the more common 6mm drill bit. Measuring 0.2362 inches, it is about 0.0018 inches larger than the 15/64″ bit. While this slight oversize makes it less ideal for tapping, it is an excellent choice for general-purpose clearance holes where a fastener needs to pass through without binding.
The Number Gauge Option: The #1 Drill Bit
In the number gauge system, the #1 drill bit is another viable alternative. It has a diameter of 0.2280 inches, which is about 0.0064 inches smaller. While not as close as the letter ‘A’ or 5.9mm bits, it can be a workable option when you need a slightly tighter pilot hole, especially in softer materials where the hole may naturally widen slightly during drilling.
If you’re facing a situation where your only alternative is a significantly smaller bit, such as a 7/32″, you may need a different strategy. Sometimes, you have to work with the tools available, and learning how to adapt when you don’t have a 7/32 drill bit can provide valuable techniques applicable to other missing sizes.
Drill Bit Equivalent Quick Reference Chart
To simplify the selection process, here is a direct comparison of the 15/64″ drill bit and its closest substitutes. The “Difference” column shows how much larger (+) or smaller (-) the equivalent is compared to the 15/64″ (0.2344″) bit.
| System | Size | Decimal Equivalent (inches) | Difference from 15/64″ (inches) | Best Use Case |
|---|---|---|---|---|
| Fractional (Target) | 15/64″ | 0.2344″ | N/A | Tap drill for 5/16″-18 thread |
| Letter Gauge | A | 0.2340″ | -0.0004″ | Excellent all-around substitute, including tapping |
| Metric | 6mm | 0.2362″ | +0.0018″ | Good for clearance holes, slightly loose fit |
| Metric | 5.9mm | 0.2323″ | -0.0021″ | Excellent for tapping, ensures tight threads |
| Letter Gauge | B | 0.2380″ | +0.0036″ | Acceptable for loose clearance holes in wood |
| Number Gauge | #1 | 0.2280″ | -0.0064″ | Workable for tight pilot holes in soft materials |
How to Select the Right Equivalent for Your Specific Task
The best equivalent is not just the one with the closest measurement; it’s the one best suited for your material and application. The difference between a successful outcome and a failed one can be just a few thousandths of an inch. Follow this decision-making guide to make the right choice every time.
For Tapping Threads in Metal
This is the least forgiving application. Your goal is to create strong, clean threads, which requires a hole that is slightly smaller than the tap’s major diameter to give the cutting edges material to work with. The ideal amount of thread engagement for most applications is around 75%.
For a 5/16″-18 tap, the 15/64″ bit is the 75% standard. Therefore, you should always choose a slightly smaller equivalent. The letter ‘A’ bit (0.2340″) or the 5.9mm bit (0.2323″) are your top choices. Never use a larger bit like the 6mm (0.2362″), as this will result in shallow, weak threads prone to stripping.
For Clearance Holes (Close Fit)
A close-fit clearance hole allows a bolt or screw to pass through with minimal play. This is important for components that need to be precisely aligned. The hole should be only slightly larger than the fastener.
If you are creating a clearance hole for a 15/64″ diameter pin or fastener, the 6mm bit (0.2362″) is an excellent choice. It provides just enough extra space for an easy fit without being excessively loose. The letter ‘B’ bit (0.2380″) can also work, but it will result in a noticeably looser fit.
For Clearance Holes (Free Fit)
A free-fit or loose-fit clearance hole is used when quick assembly is more important than precise alignment. This is common in general woodworking and some types of metal fabrication. The goal is to create a hole large enough for the fastener to pass through easily, even if there is slight misalignment between parts.
Here, the 6mm or even the letter ‘B’ bit would be perfectly acceptable. The slight oversize makes lining up the holes and inserting the fasteners much easier. For a 1/4″ screw (0.250″), a 15/64″ hole is often too tight, so a slightly larger equivalent may be beneficial.
The Material Matters: Adjusting for Wood vs. Metal
The material you are drilling into plays a significant role in how the drill bit behaves and what the final hole size will be. Wood fibers tend to compress and tear, while metal cuts cleanly. This distinction impacts your choice of an equivalent.
In most types of wood, especially softwoods, there is more forgiveness. Using a slightly oversized bit like the 6mm for a clearance hole is rarely an issue. Even a slightly undersized bit may work for screws, as the threads will bite into the wood fibers to secure themselves.
In metals, particularly hard steels, precision is paramount. Metal does not compress like wood. A drill bit can also “wander” or deflect slightly upon starting, potentially resulting in a slightly oversized hole. For this reason, always err on the side of a smaller equivalent, like the letter ‘A’ bit, for any critical application in metal.
The Hidden Factor: Heat Expansion in Metal Drilling
An often-overlooked aspect of drilling metal is the effect of heat. The friction between the drill bit and the workpiece generates significant heat, causing both the bit and the surrounding metal to expand. This means a 6mm bit, when hot, might actually drill a hole that is 6.1mm or larger.
For high-precision tasks, this thermal expansion must be managed. Using cutting fluid helps to cool the bit and workpiece, minimizing expansion. Furthermore, starting with a slightly undersized bit, like the 5.9mm or letter ‘A’, can compensate for this effect, resulting in a hole that is perfectly sized once it cools down.
When an Equivalent Isn’t Good Enough: Finding the Real Thing
While an equivalent can solve the problem in most cases, some projects demand the exact size. For high-tolerance aerospace, automotive, or tool-making applications, a substitute may not be an option. In these situations, you will need to acquire an actual 15/64″ drill bit.
Fortunately, they are not impossible to find. While they may not be in a standard 29-piece set, they are readily available for individual purchase. Finding specialized hardware often just means knowing where to look. Much like knowing where to find the right IKEA screws at Home Depot, locating a specific drill bit is easy once you check the right places.
Look in the specialty drawers or individual bit displays at large hardware stores. If local stores don’t have it, online industrial suppliers like McMaster-Carr, Grainger, and Amazon are guaranteed to have them in various materials and coatings, such as black oxide, cobalt, and titanium nitride (TiN) for enhanced durability.
The Long-Term Solution: Invest in a Comprehensive Drill Bit Index
To prevent this issue from happening again, the best long-term solution is to invest in a complete drill bit index set. A 115-piece set is an excellent investment for any serious DIYer or professional. These sets typically include the full range of fractional sizes (from 1/16″ to 1/2″), all number gauge sizes (#1 to #60), and all letter gauge sizes (A to Z), ensuring you will always have the exact bit for any job.
Conclusion: Drill with Confidence
Being stopped by a missing 15/64″ drill bit is a temporary problem, not a permanent roadblock. By understanding the precise dimensions and the available equivalents in the letter, number, and metric systems, you can almost always find a suitable replacement in your existing collection. The letter ‘A’ drill bit stands out as a nearly perfect match, while the 5.9mm and 6mm bits offer excellent options for tapping and clearance holes, respectively.
The key to success lies in analyzing your specific application. Consider the material, the type of hole needed, and the required tolerance. By matching the right equivalent to the task at hand, you can ensure a professional, durable, and precise result for your project, turning a moment of frustration into an opportunity to apply your technical knowledge.
