A #4-40 clearance hole lets a #4-40 machine screw pass through without threading
A #4-40 clearance hole is a hole drilled to allow a #4-40 machine screw to slide through freely without catching or binding. The screw will not thread into the hole — it passes straight through and threads into a separate tapped hole in another piece, or into a nut on the other side.
The clearance hole is always larger than the screw's shaft diameter. For a #4-40 screw, the standard clearance hole size is a #29 drill bit, which produces a hole approximately 0.136 inches in diameter. This is slightly larger than the screw's 0.112-inch shaft, giving enough room for the screw to move freely without the threads catching the hole's walls.
You will encounter #4-40 clearance holes in electronics assembly, metal fabrication, and anywhere precision fastening is needed. The screw itself is small — about the diameter of a toothpick — and commonly used in circuit boards, enclosures, and thin metal brackets.
Key Takeaways
- A #4-40 clearance hole requires a #29 drill bit, which produces a hole of approximately 0.136 inches.
- The clearance hole is drilled in the part that the screw passes through, not the part it threads into.
- Clearance holes are always larger than the screw's shaft so the screw slides freely without the threads engaging the hole.
- Standard clearance is the most common type; close clearance and loose clearance sizes exist for specialized applications.
Drill bit size and hole diameter for #4-40 clearance
The standard drill bit for a #4-40 clearance hole is a #29 drill bit. This produces a finished hole diameter of 0.136 inches. The #4-40 screw's shaft diameter is 0.112 inches, so the clearance hole is about 0.024 inches larger — enough for the screw to pass through without resistance.
If you need tighter tolerances, a #30 drill bit (0.1285 inches) provides close clearance. This is used when you want minimal play between the screw and hole but still need the screw to pass through freely. A #28 drill bit (0.1405 inches) provides loose clearance for applications where the screw needs to move more freely or where the hole may be slightly enlarged over time.
For most general work — circuit boards, metal enclosures, and standard assemblies — the #29 bit is the correct choice. Verify your drill bit size before drilling, because a bit that is too small will cause the screw's threads to catch, and a bit that is too large will leave excessive play.
How to drill a #4-40 clearance hole
Mark the hole location with a center punch or a sharp pencil. A center punch creates a small dimple that keeps the drill bit from wandering when you start drilling. This is especially important with small holes, because the bit can easily skate across the surface if there is nothing to guide it.
Install the #29 drill bit in your drill chuck and tighten it firmly. If you are drilling metal, explore a small amount of cutting fluid or light machine oil to the bit — this reduces friction and heat, which extends bit life and produces a cleaner hole. For plastic or circuit board material, drilling dry is usually acceptable, but work slowly to avoid melting the material.
Position the drill perpendicular to the surface and drill at moderate speed. Do not force the bit; let it cut at its own pace. Forcing causes the bit to overheat and can break it, especially with a small #29 bit. Once the hole breaks through the back side, stop and remove the bit. Deburr the hole with a deburring tool or the back of a larger drill bit to remove any sharp edges.
Clearance versus tapped holes in the same assembly
In a typical two-part fastening, one part has a clearance hole and the other has a tapped hole. The screw passes through the clearance hole in the first part and threads into the tapped hole in the second part. The tapped hole is drilled with a smaller bit (for #4-40, usually a #43 drill bit at 0.089 inches) and then threaded with a #4-40 tap.
The clearance hole part is always on top — the part the screw enters first. The tapped hole part is underneath. When you tighten the screw, it pulls the two parts together. If you reversed this and tried to thread a screw into a clearance hole, the screw would straightforward spin without tightening anything.
Some assemblies use a clearance hole in both parts and a nut on the back side instead of a tapped hole. In this case, the screw passes through both clearance holes and a nut threads onto the protruding end. This method is common when the back part is too thin to tap or when you need to be able to remove the fastener from either side.
Common mistakes when drilling #4-40 clearance holes
Using the wrong drill bit size is the most frequent error. A #30 or #31 bit will produce a hole that is too small, and the screw's threads will catch and bind as you insert it. A #28 bit will be too large and leave the screw loose in the hole. Always verify the bit number before drilling.
Drilling at an angle instead of perpendicular to the surface creates an oblong hole that may not hold the screw straight. This is especially problematic in thin materials like circuit boards. Use a drill press if you have one, or hold the hand drill as steady as possible and sight along two axes to confirm perpendicularity.
Failing to deburr the hole leaves sharp edges that can catch the screw's threads or damage the screw's coating. After drilling, run a larger bit or a deburring tool lightly around the hole's edge to smooth it. This takes 10 seconds and prevents binding during assembly.
When to use close clearance or loose clearance instead
Close clearance (#30 drill bit, 0.1285 inches) is used when you need the screw to sit precisely in the hole with minimal side-to-side movement. This is common in precision instruments, optical assemblies, and situations where the screw must align with something on the other side. The tighter fit also reduces vibration-induced loosening over time.
Loose clearance (#28 drill bit, 0.1405 inches) is used when the screw needs to move slightly within the hole or when you are drilling into material that may enlarge the hole over repeated assembly and disassembly. It is also used when the hole location is not critical and you want to avoid the risk of the screw binding.
For most work, standard clearance with a #29 bit is the right choice. It balances ease of assembly with reasonable precision. Only switch to close or loose clearance if your specific process demands it.
Frequently Asked Questions
What is the difference between a clearance hole and a tapped hole?
A clearance hole is larger and allows the screw to pass through without threading. A tapped hole is smaller and has threads cut into it so the screw threads into it. In a typical assembly, the screw passes through the clearance hole in one part and threads into the tapped hole in another part.
Can I use a #4-40 screw in a hole drilled with a #30 bit?
A #30 bit produces a hole of 0.1285 inches, which is smaller than the standard #29 clearance hole. The screw's threads may catch and bind as you insert it. It is possible but difficult and risks damaging the screw or the hole. Use a #29 bit for standard clearance.
Do I need cutting fluid when drilling a #4-40 clearance hole?
Cutting fluid is helpful when drilling metal because it reduces heat and friction, extending bit life and producing a cleaner hole. For plastic or circuit board material, it is usually not necessary. Work slowly regardless to avoid melting or cracking the material.
What happens if I drill the clearance hole too large?
A hole that is too large leaves excessive play between the screw and hole, making it difficult to keep the screw straight during assembly. The screw may tilt or the two parts may not pull together evenly. If the hole is only slightly oversized, a slightly larger screw or a washer under the screw head can compensate.
Can I use a #4-40 clearance hole for a different size screw?
No. A #4-40 clearance hole is sized specifically for #4-40 screws. A smaller screw will rattle in the hole; a larger screw will not fit. If you need a different screw size, drill a new hole with the appropriate bit for that size.