Drill Bit Size For 8-40 Screw: A Perfect Fit (2025)

When it comes to selecting the right drill bit size for an #8 screw, there are several factors to consider, including the screw type, the materials used, and the thread design. For instance, the recommended drill bit size for a #27 pilot hole is for #8 hammer drive or U-drive screws. Meanwhile, a #30 drill bit is suggested for high-low thread-forming screws and materials with a flexural modulus of more than 200,000 PSI. If the materials rate below this figure, a #31 drill bit is a better option. For Type A, AB, or B self-tapping screws or Type 25 thread-cutting screws, a #29 drill bit is required.

Drill bit sizes for #8 screws

Drill bits are cutting tools used in drilling machines and are available in various sizes. The #8 screw is a common size used in multiple applications. Here is a detailed guide on selecting the right drill bit size for #8 screws.

Understanding Drill Bit Sizing

Drill bits are sized using different naming conventions, including fractions, numbers, and letters. In the US, fractional inch and gauge drill bit sizes are commonly used, while metric sizes are the standard in most other countries. The sizes increase from #80 through #1, followed by letter sizes from A to Z. The gauge-to-diameter ratio is based on the Stubs Steel Wire Gauge, which originated in 19th-century Britain.

Selecting the Right Drill Bit for #8 Screws

For #8 screws, the recommended drill bit size is #21 to #25, depending on the thread type. The exact size can vary depending on the specific type of #8 screw you are using. It's important to match the correct drill bit size with the screw to ensure a secure fit and avoid damage to the material.

Considerations for Different Screw Types

When working with Type A self-tapping screws, you may need to use a slightly different drill bit size compared to other #8 screw variants. Additionally, the material being drilled can also impact the recommended drill bit size. For example, drilling into wood may require a different size than drilling into metal.

Pilot Hole Drilling

For #8 screws, it is generally recommended to drill a pilot hole before inserting the screw. The pilot hole ensures that the screw can be driven smoothly without splitting the material. The pilot hole size should match the recommended drill bit size for your specific #8 screw.

In conclusion, selecting the appropriate drill bit size for #8 screws is crucial for successful fastening. By understanding the sizing conventions, considering screw types, and drilling appropriate pilot holes, you can achieve secure and damage-free results in your projects.

Pilot hole size for softwood

A pilot hole is a small hole drilled into a piece of construction material to guide a larger drill or the insertion of a screw. Pilot holes are often used with screws to prevent the wood from splitting and to ensure the screw does not break.

When drilling a pilot hole for an 8-40 screw in softwood, the drill bit should be the same diameter as the shank of the screw. This will ensure the screw threads can cut directly into the hole, reducing the risk of the wood splitting. The pilot hole should be the same depth as the screw you will be using.

The process of drilling a pilot hole begins with selecting the right drill bit. In this case, the drill bit should match the inner diameter of the threaded part of the screw. The location of the pilot hole should then be measured and marked. This involves measuring across the workpiece and making short lines at the intersection point, which will be the centre point for the pilot hole.

Once the location is marked, the drill bit should be installed in the chuck—the part of the drill that holds the bit in place. It is important to ensure the bit is centred and the chuck is tightened completely. The drill/driver's setting should then be turned to drill mode, and eye protection should be worn.

With the drill prepared, the pilot hole can be drilled. The drill should be aligned perpendicular to the surface, using a speed square as a guide. It is best to start slowly and then increase the speed.

While pilot holes are generally recommended for harder materials and dense types of wood, they can also be useful for softwoods like pine and spruce if precise screw locations are needed or if the screw is being driven through the edge or end of the material.

Pilot hole size for hardwood

Pilot holes are important when installing wood screws. They make it easier to drive in the screw, prevent the wood from splitting, and help ensure the screw is in the right place. If the pilot hole is too large, the screw's threads won't hold as well. If it's too small, it will be easy to strip out a screw's head or split the wood.

When drilling into hardwood, the general rule is to use a slightly larger drill bit than you would for softwood. For example, when using traditional slotted or Phillips drive wood screws, a tapered shank drill bit is required, as it fits the profile of the screw almost perfectly, providing maximum holding power.

The size of the pilot hole will depend on the specific screw you are using. For an 8-40 screw, the recommended pilot hole diameter is 0.0990 inches (or 2.5146 millimetres). This is based on the assumption that the screw has a standard thread pitch of 0.0125 inches. If your screw has a different thread pitch, the recommended pilot hole size may be slightly different.

It's important to note that the recommended pilot hole size may vary depending on the type of wood screw and the specific application. Always refer to the manufacturer's instructions or consult a specialist for the most accurate information.

Drill bit sizes for plastics

To answer your first question, a #41 or #42 drill bit is recommended for an 8-40 screw.

Now, here is some information about drill bit sizes for plastics.

When drilling into plastic, it is important to use the correct drill bits to avoid chipping or cracking the material. Drill bits designed for plastic are available in various sizes, and it is recommended to consult a professional before starting a project. The larger the hole you are drilling, the slower the drill speed should be to avoid melting the plastic. Always reduce the drill speed as the drill bit exits the material, and clamp down the plastic to a solid surface before drilling. Using a lubricant will help remove debris and reduce heat. As plastic expands and contracts with temperature changes, it is important to drill a slightly larger hole than necessary to allow room for screw movement and avoid placing stress on the material.

Spur-point bits, also known as dowel bits, are specifically angled to make smooth cuts in plastic. They have a central point and spurs that keep the bit aligned, resulting in a clean, neat hole. Brad point drill bits are another option, as they have a sharp centre point that keeps the bit positioned as it enters the material, creating cleaner, more precise holes.

For softer plastics, auger drill bits are ideal. They have a screw-like design that moves the material out of the way, resulting in less cleanup. Step drill bits are also versatile and efficient, as they can create multiple hole sizes with just one bit. If you are drilling into harder plastics or ultra-hard materials, consider using carbide drill bits or diamond-coated drill bits, which are the most durable option.

For an 8-40 screw, a #40 drill bit is required. This is a standard size in the US, where drill bits are measured in fractional inches. In the US, you'll also find gauge drill bit sizes.

Drill bits are the cutting tools of drilling machines. They can be made-to-order in any size, but standards organisations have defined sets of sizes that are routinely produced and stocked.

In the US, fractional inch and gauge drill bit sizes are in common use. In nearly all other countries, metric drill bit sizes are most common.

Fractional-inch drill bit sizes are written as irreducible fractions. So, instead of 78/64 inch, the size is noted as 1 7/32 inch.

Number and letter gauge drill bit sizes range from size 80 (smallest) to size 1 (largest), followed by letter gauge size A (smallest) to size Z (largest).

Metric drill bit sizes define the diameter of the bit in terms of standard metric lengths. For example, British Standard BS 328 defines 230 sizes from 0.2mm to 25.0mm.

Frequently asked questions