Clearance Holes: Metric & Imperial Sizes
- Gavin Leo
Clearance holes are essential when assembling components. They allow for easy insertion of fasteners and provide room for the fastener to move slightly during the assembly process or due to thermal expansion and contraction.
Clearance hole sizes refer to the diameters of holes that are designed to provide a clearance or free space around a fastener such as a bolt or a screw.
Clearance Hole Chart for Metric Bolts
The chart below helps in understanding the dimensions of these threads, detailing major and minor diameters for both external and internal threads.
| Nominal Diameter (mm) | Close Fit (mm) | Normal Fit (mm) | Loose Fit (mm) |
| 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 | 2.0 |
| 1.8 | 2 | 2.1 | 2.2 |
| 2 | 2.2 | 2.4 | 2.6 |
| 2.2 | 2.4 | 2.6 | 2.8 |
| 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 |
| 8 | 8.4 | 9 | 10 |
| 12 | 13 | 14 | 15 |
| 16 | 17 | 18 | 19 |
| 20 | 21 | 22 | 24 |
| 24 | 25 | 26 | 28 |
| 30 | 31 | 32 | 35 |
| 33 | 34 | 36 | 38 |
| 36 | 37 | 39 | 42 |
| 39 | 40 | 42 | 45 |
| 45 | 46 | 48 | 52 |
| 52 | 54 | 56 | 62 |
| 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 | 95 | 101 | 107 |
| 100 | 104 | 107 | 112 |
| 115 | 119 | 122 | 127 |
| 120 | 124 | 127 | 132 |
| 125 | 129 | 132 | 137 |
| 130 | 134 | 137 | 144 |
Clearance Hole Chart for Imperial Bolts
Clearance Hole Drills
Size of Screw
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 |
How to measure clearance hole size accurately?
The chart below helps in understanding the dimensions of these threads, detailing major and minor diameters for both external and internal threads.
Using Precision Measuring Tools
Calipers: Vernier calipers or digital calipers are excellent tools for measuring the diameter of a clearance hole. The jaws of the calipers are inserted into the hole, and the measurement is taken directly from the scale on the caliper. Digital calipers provide a more precise reading, often to an accuracy of 0.01 mm or 0.001 inches, depending on the model.
For example, when measuring a metric clearance hole for a small mechanical part, a digital caliper can accurately measure the diameter to ensure it meets the required specifications.
Micrometers: For very precise measurements, especially for smaller holes, inside micrometers can be used. These tools have a spindle and an anvil that are inserted into the hole, and the measurement is obtained by adjusting the spindle until it contacts the hole wall. The reading is then taken from the micrometer scale.
Inside micrometers are often used in high – precision manufacturing, such as in the production of watch components or precision-engineered aerospace parts.
Optical Measurement Methods
Microscopes: Optical microscopes with measurement capabilities can be used to measure the diameter of clearance holes. The hole is magnified under the microscope, and a calibrated scale in the microscope’s field of view is used to measure the diameter.
This method is useful for very small holes or when a high – magnification view is needed to assess the accuracy of the hole’s shape and size. For example, in the electronics industry, when measuring the clearance holes for micro – sized components on a circuit board, an optical microscope can provide accurate measurements.
Vision Measuring Systems: These are more advanced optical measurement tools that use cameras and software to measure the dimensions of objects. The hole is illuminated, and the camera captures an image of it.
The software then analyzes the image and calculates the diameter of the hole. Vision measuring systems are often used in automated manufacturing processes where a large number of parts need to be measured quickly and accurately.
Gauge Pins and Plug Gauges
Gauge Pins: These are precision – machined pins with known diameters. The smallest gauge pin that can be inserted into the clearance hole without force gives an indication of the lower limit of the hole diameter. This method is simple and effective, especially for quickly checking whether a hole is within a certain size range.
For example, in a workshop environment, gauge pins can be used to quickly verify that the clearance holes on a batch of machined parts are of the correct size before further assembly.
Plug Gauges: Plug gauges are similar to gauge pins but are usually more complex in design. They have a go – no – go feature, where one end of the plug gauge (the “go” end) should fit into the hole if the hole is within the acceptable size range, and the other end (the “no – go” end) should not fit.
This provides a clear indication of whether the hole is within the specified tolerance. Plug gauges are commonly used in quality control processes for mass – produced parts.
Coordinate Measuring Machines (CMMs)
CMMs are highly accurate and versatile measurement devices. The part with the clearance hole is placed on the CMM’s measuring table. A probe on the CMM is then used to touch various points on the edge of the hole. The CMM’s software calculates the diameter of the hole based on the coordinates of these points.
CMMs can measure not only the diameter but also the position and orientation of the hole in three – dimensional space. They are widely used in industries such as automotive and aerospace, where high – precision measurement of complex parts with multiple holes is required.
Factors Affecting Clearance Hole Size
Several factors can influence the size of a clearance hole, including the type of material, operating temperature, and the desired level of precision. For instance, materials prone to thermal expansion may require larger clearance holes to accommodate changes in size due to temperature fluctuations. In high-precision applications, smaller clearance holes may be necessary to ensure a snug fit and proper function of the threaded components.
The type of thread also plays a role in determining the clearance hole size. Coarse threads, which have fewer threads per inch, typically require larger clearance holes compared to fine threads. This is because coarse threads are designed for quick assembly and disassembly, while fine threads are used in applications requiring a tighter fit and greater holding strength.
Understanding these factors is essential for selecting the correct threaded components and tools for your project, ensuring that everything fits together perfectly and functions as intended.