How to Measure an Automotive Screw, Bolt, Nut and Thread?
Do mismatched fastener measurements cause assembly problems or slow down the manufacturing process? Precision is essential, as even minor errors can have serious effects.
With years of fastener manufacturing experience, we have developed measurement techniques that save time and prevent flaws.
Continue reading this guide for tips on obtaining nuts, bolts, and screw measurements.
Screw Measurements: How to Measure Screw Size and Dimensions of a Screw?
You have to examine length, diameter, and thread pitch in order to get the accurate size and dimension of a screw.
Below is the detailed and easy guide for how to do automotive screw measurements:
- Screw Length
The length is usually calculated by measuring it from the tip to the bottom of the screw head. However, here are some considerations for precise screw measurements:
- Rounded screws: from the flat part underneath the screw’s head to its tip.
- Hex washer and hex head screws: From the bottom of its head (below the hexagonal shape of the screw) to the screw’s tip.
- Screws with flat tops like countersunk: its flat top surface to its end.
Use a measuring tape to place the screw flat against it. A caliper can provide more accurate results. Then, using the list above, measure it in inches or millimeters, depending on whether it is a metric or imperial screw.
- Screw Diameter
In screw measurements, this refers to the width, which is defined by two types of diameters:
- Major diameter: This is the peak or the highest point of the screw’s threads.
- Minor diameter: The lowest screw or bolt diameter, as measured at the external thread’s bottom edges or roots.
For minor diameter, the measuring tape or digital caliper must touch the screw at its lowest position, whereas for major diameter, it should be aligned at its highest point.
In all automotive fasteners, not just screw measurements, millimeters should be used if the screw is metric and inches if it is an imperial screw.
- Pitch of Screw Thread: How to measure it?
The most difficult screw measurements are the number and spaces between the threads, often known as pitch. It is measured in threads per inch (TPI) for imperial screws and in millimeters for metric screws.
Align an imperial screw with a ruler and carefully count the number of threads in one inch to determine the pitch. Measure the distance between threads’ peaks if the screw is metric.
How to Read Screw Measurements?
The correct screw measurement depends on whether the screw is metric or imperial.
Metric measurements are labeled with three numbers: the first represents the thread’s major diameter in millimeters, the second is the thread pitch, and the third is the length. For example, M4.5-1.0 x 40 mm specifies that the screw’s main diameter is 4.5 mm and its length is 40 mm while the pitch is 1.0.
Imperial measurements have a format like 1/4″-20 x 2″. The fraction or first number represents the screw’s major diameter in inches. The second determines the threads per inch, while the final one defines the screw’s overall length in inches.
Bolt Measurements: How to Measure Bolt Size?
- Bolt Grade
Search for markings in the bolt head and count them. These markings might be lines or decimal numbers to determine their material type and strength.
We provided a youtube video for reference and made the identification of the bolt’s material properties easier.
- Thread Pitch
For metric bolts, measure the distance from the peak of one thread to the next, while for imperial bolts, count the number of threads in an inch (TPI).
If the shank is smaller than one inch, divide the number of threads counted by the length.
For example, your bolts’ threaded section is only 0.75 inch, and you count 6 threads on it. Divide 0.75 by the thread count, which is 6 in our case. As a result, our bolt’s TPI is 8.
- Shank Diameter
Using a caliper or a bolt gauge, align the widest unthreaded section or shank of the bolts to determine their width.
- Length of the Shank
For standard bolts, measure from under the head to the tip. If you’re measuring countersunk bolts, start from the top of the head and work your way down to the tip.
- Head Size and Shape
Figuring these out is crucial for selecting proper tools, such as wrenches for bolts.
Using a caliper, measure the width and thickness of the head, as well as whether it is square or another shape.
- Measure Thread Length
Begin measuring from where the threads start to where they end.
How to Calculate Root Length in a Bolt?
To find the root length of a bolt, use the formula “D – 1.226869 x P.”.
First, establish the bolt’s major diameter (D) and thread pitch (P). If you have imperial bolts, divide the TPI by 1 and multiply by 25.4 to get the pitch in millimeters.
Then apply the formula to get the root length, rounding up if the number following the decimal point is greater than 5.
Nut measurements: How to measure nut size?
- Diameter
The diameter of the nut refers to its center and determines the size of bolt that will fit through it.
So, using a calliper or a thread gauge, calculate the nut’s internal diameter.
Make sure you’ve measured across the nut’s inner section or axis.
- Thread Pitch
Similar to others, use thread pitch gauge to figure out the pitch.
If you don’t have one, depend on the number of threads in one inch for imperial.
If it’s a metric screw, measure the gap between multiple threads and divide by the number of spaces to calculate the pitch in millimeters.
- Measure Across Flats
Take a caliper or ruler and measure from one flat side to the opposite flattened exterior to determine the wrench size you need.
When using a ruler, ensure it is marked in appropriate units to your screw type, which is either imperial or metric.
- Determine Thickness
When positioned flat, the thickness of the nuts corresponds to their height.
Use a ruler or caliper to determine the length from top to bottom.
When using a ruler, be certain that it is vertically aligned from the flat surface to the nut’s top.
How to Determine Thread Size?
Major Diameter
Measure the highest point (the crest) of the automotive fastener’s thread with a caliper.
For automotive fasteners with interior threads like nuts, use a caliper for precise measurements.
However, if you don’t have a caliper and the nut’s hole is too small for a ruler to fit, use a telescoping gauge.
Place the gauge in the hole, then slowly expand the plungers by loosening the lock until they are against the nut’s side. To keep the plungers in place, tilt the gauge slightly (15 degrees) and tighten the locking screw.
Remove it carefully from the nut to measure across the plunger’s ends with a micrometer.
Identify Thread Type
Refer to the thread identification chart and look for the standard thread sizes and types that fit your measurements.
For example, if you measure a bolt with a diameter of 10 mm and a pitch of 1.5, you can use the chart to determine that it is M10 x 1.5 for metric or #10-20 for imperial.
This is an additional informational video to assist you in precisely measuring the thread size and identifying them.
Determine Threads Per Inch (TPI) or Pitch
For imperial threads, place a ruler into the threaded portion of the fastener and count how many highest points (crests) are within one inch.
However, if the fastener is less than one inch long, multiply the total length of your threaded portions by the total number of threads you counted.
For metric threads, measure the distance between numerous threads’ highest point in millimeters before dividing it by how many threads you found to accurately get the pitch.
Measurement Techniques for Non-Standard Automotive Fasteners
To measure non-standard fasteners, identify their type (like a bolt or screw) as well as their specifications, such as material, size, diameter, pitch, and length.
Measure the width of the threads by measuring their outer diameter with a caliper. Likewise, metric fasteners are measured in millimeters, but imperial fasteners are in inches.
Next, to get pitch, count the number of threads if it’s an imperial fastener, but measure the distance between the threads for metric.
Furthermore, calculate how long the fastener is from its head to the tip with rulers or calipers, as well as look for additional properties to ensure it fits your requirements, including:
- Thread type
- Special coatings
- Integrated features such as washers
In addition to the basic measurements, you may evaluate its performance using additional tests such as:
- Torsion testing: How much torque or twisting force can a fastener withstand before breaking or deforming.
- Hardness testing: Determines a fastener’s resistance to distortion or damage when exposed to applied force.
- Non-destructive testing: utilizes ultrasonic testing, magnetic particle testing, or dye penetrant testing to detect flaws in a fastener without causing damage.
FAQs
- How do I measure pitch diameter?
- To calculate the external threads, multiply the thread pitch by 0.6495 and subtract it from the main diameter. Meanwhile, for internal threads, multiply 0.4330 by the thread pitch before adding it to the minor diameter.
- However, thread plug gauges or thread ring gauges are advised for a more convenient and precise measurement method.
- How do you measure nuts and bolts?
- To get the bolts’ measurements, calculate their shank diameter, thread pitch, length, head size, and shape, as well as the bolt markings.
- For nuts, determine the inner diameter, across the flats, thickness, and thread pitch. You can also find their markings to figure out their grade and type.
- What are the dimensions of a screw?
- The dimensions of a screw are diameter, length, and thread pitch, which are all essential for precise screw measurements and effectively determine its type.
Resources
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