When building or fixing things with screws and bolts, it’s very important to make the correct holes for the screws or bolts to fit properly. Clearance holes help screws and bolts fit through parts easily and securely without damaging the materials. Using the right type and size of hole ensures that the parts are tightly joined, safe, and strong. In this article, we’ll explain what clearance holes are, how they differ from other types of holes like tapped holes and pilot holes, and how to calculate and find the right diameter using formulas and metric/imperial bolt clearance hole charts.
What Is a Clearance Hole?
A clearance hole is a hole drilled through a piece of material that is just a little bigger than the diameter of the bolt or screw shaft (the smooth part without threads). This allows the bolt to slide through the hole easily without the threads catching or biting into the material around the hole. The hole is still small enough so that the bolt head cannot pass through, which helps hold the parts together tightly.
Clearance holes are very useful when you want to join two or more parts together. The bolt passes freely through the clearance hole in the first part and then screws into the second part or a nut. This way, the bolt pulls the parts snugly together without damaging the first part by the threads digging in. Clearance holes make assembling parts faster and prevent problems like material splitting or bolts getting stuck.
Clearance Holes vs Tapped Holes: What Are the Differences?
1. Hole Type
A clearance hole is a smooth hole without any threads inside it. It is drilled larger than the bolt shaft to let the bolt pass through freely. On the other hand, a tapped hole is a hole where threads have been cut or tapped inside the hole walls. These threads match the bolt’s threads so the bolt can screw directly into the material itself without needing a nut.
2. How the Bolt Is Held
In a clearance hole, the bolt is held tightly by the nut or by threads in another part behind the hole, not by the hole itself. This means the clearance hole only provides space for the bolt to pass through. In contrast, a tapped hole holds the bolt directly because the threads inside the hole grip the bolt’s threads tightly, holding it in place without a nut.
3. Strength and Permanence
Tapped holes usually provide a stronger and more permanent connection because the bolt is screwed directly into the material, making the joint very secure. Clearance holes depend on the nut or another threaded part for strength. Clearance holes are great for parts that need to be taken apart or adjusted easily, while tapped holes are better for parts that should stay fixed permanently.
4. Application and Accessibility
Clearance holes are often used when you can access both sides of the parts being joined because you need to use a nut or threaded part behind the hole. Tapped holes are useful when you cannot reach the back side of the material to attach a nut, such as in thin sheet metal or machine parts, so you tap threads inside the hole itself.
Which One to Choose?
If your project allows access to both sides of the parts and you want easy assembly or disassembly, go with clearance holes. They make putting things together faster and simpler, especially when you need to replace parts. However, if you want a strong, permanent joint and can’t reach the back side to use a nut, a tapped hole is better. It provides a solid hold by threading into the material directly. Always consider the material type, access, and how permanent the joint needs to be when deciding between clearance and tapped holes.
Pilot Hole vs Clearance Hole: What Are the Differences?
1. Purpose of the Hole
A pilot hole is a small, guiding hole drilled into a material before inserting a screw or bolt. Its main purpose is to make it easier to drive the screw straight and to prevent the material, especially wood, from splitting when the screw is inserted. A clearance hole, however, is made larger than the screw shaft so the screw can freely pass through one part without gripping it, allowing the bolt to pull two parts tightly together.
2. Size of the Hole
Pilot holes are smaller than the outer diameter of the screw threads. This ensures there is still material around the hole for the threads to grip and hold the screw in place. Clearance holes are bigger than the screw shaft diameter but smaller than the screw head diameter, so the screw threads do not engage with the first piece but only with the second piece or a nut.
3. How They Affect Assembly
Pilot holes help the screw enter the material straight and reduce the chance of cracking or splitting, but they do not allow parts to move freely. Clearance holes, by contrast, let the screw or bolt pass through one piece without resistance so the parts can be pulled tightly together by the threads engaging only in the second piece or nut.
4 . When to Use Each
Use pilot holes when you need to drive screws into hard materials like hardwood or plastics where the risk of splitting or breaking the screw is high. You also use pilot holes to guide screws for precise, clean assembly. Clearance holes are used when you want the screw or bolt to pass freely through the first piece and tighten against the second piece, like when joining two planks with bolts.
What Is the Clearance Hole for an M4 Bolt?
For an M4 bolt (4 mm diameter shaft), the clearance hole diameter is generally 4.5 mm. This size ensures the bolt can pass through easily without the threads catching on the hole edges. For a tighter or closer fit, a clearance hole of 4.3 mm can be used, which allows less play but still lets the bolt pass through cleanly.
What Is Clearance Hole Diameter, and How to Calculate It?
The clearance hole diameter is the size of the hole you need to drill so the bolt or screw can pass through freely without the threads biting into the hole edges or getting stuck. To calculate the correct clearance hole size, use this simple formula:
Clearance Hole Formula: C = (DF + DH)/2
Where:
- C = Clearance Hole diameter
- DF = Diameter of the fastener (bolt or screw shaft)
- DH = Diameter of the fastener’s head (the widest part of the bolt or screw head)
Steps to Calculate:
- Find the diameter of the bolt shaft (DF).
- Measure or look up the bolt diameter.
- Find the diameter of the bolt head (DH).
- Measure the widest part of the bolt or screw head.
- Apply the formula: Add DF and DH, then divide the sum by 2.
- Get the clearance hole size: This will be the diameter of the hole to drill.
Example:
Bolt shaft diameter (DF) = 1.25 inches
Bolt head diameter (DH) = 0.4 inches
Calculate:
C=(1.25+0.4)/2=0.825 inches
So, the clearance hole diameter should be 0.825 inches.
Metric Bolt Clearance Hole Size Chart
| Size – Nominal Diameter (mm) | Clearance Hole (mm) | ||
|---|---|---|---|
| Close Fit | Normal Fit | Loose Fit | |
| 1 | 1.1 | 1.2 | 1.3 |
| 1.2 | 1.3 | 1.4 | 1.5 |
| 1.4 | 1.5 | 1.6 | 1.8 |
| 1.6 | 1.7 | 1.8 | 2 |
| 1.8 | 2 | 2.1 | 2.2 |
| 2 | 2.2 | 2.4 | 2.6 |
| 2.2 | 2.4 | 2.6 | 2.8 |
| 2.5 | 2.7 | 2.8 | 3.1 |
| 3 | 3.2 | 3.35 | 3.6 |
| 3.5 | 3.7 | 3.9 | 4.2 |
| 4 | 4.3 | 4.5 | 4.8 |
| 4.5 | 4.8 | 5 | 5.3 |
| 5 | 5.3 | 5.5 | 5.8 |
| 6 | 6.4 | 6.5 | 7 |
| 7 | 7.4 | 7.8 | 8 |
| 8 | 8.4 | 9 | 10 |
| 10 | 10.5 | 11 | 12 |
| 12 | 13 | 14 | 15 |
| 16 | 17 | 18 | 19 |
| 20 | 21 | 22 | 24 |
| 24 | 25 | 26 | 28 |
| 30 | 31 | 32 | 35 |
| 30 | 31 | 33 | 35 |
| 33 | 34 | 36 | 38 |
| 36 | 37 | 39 | 42 |
| 39 | 40 | 42 | 45 |
| 42 | 43 | 45 | 48 |
| 45 | 46 | 48 | 52 |
| 48 | 50 | 52 | 56 |
| 52 | 54 | 56 | 62 |
| 56 | 58 | 62 | 66 |
| 60 | 62 | 66 | 70 |
| 64 | 66 | 70 | 74 |
| 68 | 70 | 74 | 78 |
| 72 | 74 | 78 | 82 |
| 76 | 78 | 82 | 86 |
| 80 | 82 | 86 | 91 |
| 85 | 87 | 91 | 96 |
| 90 | 93 | 96 | 101 |
| 95 | 98 | 101 | 107 |
| 100 | 104 | 107 | 112 |
| 105 | 109 | 112 | 117 |
| 110 | 114 | 117 | 122 |
| 115 | 119 | 122 | 127 |
| 120 | 124 | 127 | 132 |
| 125 | 129 | 132 | 137 |
| 130 | 134 | 137 | 144 |
| 140 | 144 | 147 | 155 |
| 150 | 155 | 158 | 165 |
Imperial Bolt Clearance Hole Size Chart
| Size of Screw | Clearance Hole Drills | ||||
|---|---|---|---|---|---|
| Close Fit | Free Fit | ||||
| No. or Diameter | Decimal Equivalent | Drill Size | Decimal Equivalent | Drill Size | Decimal Equivalent |
| 0 | 0.06 | 52 | 0.0635 | 50 | 0.07 |
| 1 | 0.073 | 48 | 0.076 | 46 | 0.081 |
| 2 | 0.086 | 43 | 0.089 | 41 | 0.096 |
| 3 | 0.099 | 37 | 0.104 | 35 | 0.11 |
| 4 | 0.112 | 32 | 0.116 | 30 | 0.1285 |
| 5 | 0.125 | 30 | 0.1285 | 29 | 0.136 |
| 6 | 0.138 | 27 | 0.144 | 25 | 0.1495 |
| 8 | 0.164 | 18 | 0.1695 | 16 | 0.177 |
| 10 | 0.19 | 9 | 0.196 | 7 | 0.201 |
| 12 | 0.216 | 2 | 0.221 | 1 | 0.228 |
| 14 | 0.242 | D | 0.246 | F | 0.257 |
| 1/4 | 0.25 | F | 0.257 | H | 0.266 |
| 5/16 | 0.3125 | P | 0.323 | Q | 0.332 |
| 3/8 | 0.375 | W | 0.386 | X | 0.397 |
| 7/16 | 0.4375 | 29/64 | 0.4531 | 15/32 | 0.4687 |
| 1/2 | 0.5 | 33/64 | 0.5156 | 17/32 | 0.5312 |