Bolt Boot Camp: 6 Things to Know About Threaded Fasteners

Sean Derrick (Product Development Engineer)

Rookie Year: The Engineering Intern’s Toolbox
A Series of Technical Blogs for Engineering Interns & Entry-Level Engineers

While warmer temps are upon us and school is out for the summer, it’s time for many to begin that most interesting season of the year… summer internships. Across the nation, engineering interns are getting their first taste of professional life and hopefully learning some basics which will serve them for years. However, some of the principles they are expected to know on day one are rarely covered in class. To help our young, budding engineers, here’s one basic lesson for their toolbox… literally the nuts and bolts.


How Do Threaded Fasteners Work?

Threaded fasteners all contain a spiraling ramp on a cylindrical shaft. This ramp is a simple mechanism for converting rotational motion into linear motion. More importantly, it converts a torque (rotational force) into a linear force. The ramp is what is referred to as a thread. When the threads are on the outside of a cylindrical shaft it is referred to as an external or male thread while a cylindrical hole with a thread is called an internal or female thread. Rotational motion is converted into linear motion by interacting (mating) an internal thread with an external thread.




How are Fasteners Meant to be Used?

Threaded fasteners are almost always intended to clamp (fasten) two or more pieces of material together. Unless specially designed to do so, threaded fasteners are not intended to locate pieces of material relative to each other or prevent pieces of material from sliding. Threaded fasteners are strongest in tension (being pulled apart) not in shear (slide apart). As a result, they prevent parts from sliding relative to each other by their clamp force not from the body of the fastener acting like a pin.




What is the Difference Between a Bolt and Screw?

This is a very confusing principle for new engineers because some bolts are screws and others are not. For example, a carriage bolt is often used as a screw and a socket head cap screw (SHCS) is often used as a bolt. The simple answer is a bolt uses a nut while a screw does not. Screws thread directly into one of the pieces of material to be clamped. In this case, the material within one of the items to be clamped acts as the internal female thread. A bolt on the other hand passes through all of the pieces of material to be clamped and does not interact with any of them. Instead, a nut or other pre-made female thread is applied to the end of the bolt and the material is clamped between the nut and head of the bolt.


What is Meant by a Fine or Coarse Thread?

Thread pitch deals with how many threads are in a given length of a fastener per its nominal diameter. For example, a fastener could have large and widely spaced threads. This is referred to as having a coarse pitch. A fastener with the same nominal diameter that has more frequent threads per unit of length has what is known as a fine pitch.Coarse threads are bigger and have more material in them. However, this means that they have a smaller non threaded cylinder section compared to fine pitch threads. Therefore, coarse pitch threads have stronger threads and weaker bodies. Fine pitch threads are the opposite which have weaker threads and stronger bodies.



When Should I Use a Fine vs. Coarse Pitch?

Most standard fasteners have a common coarse and fine pitch thread size. Coarse pitch threads are ideal when threading into a base material which is equal to or stronger then the fastener. As stated earlier the extra material in the coarse pitch threads means that the threads are stronger then fine pitch. This means they are better at being installed and removed frequently without damaging the threads. Therefore, it is better to use coarse pitch threads in soft materials when the fastener will be repetitively removed and reinstalled.Fine pitch threads give you more threads per length and therefore more load to be distributed into the base material. This is ideal if you are working with thin materials or when the base materials are softer than the fastener. Also, because the fastener’s shank is larger, the fine pitch threads have higher tensile (pulling) strength. The trouble with fine threads are that the threads themselves are much weaker. Repeatedly taking the fastener in and out will damage the threads. The other issue is that it is easy to “cross thread” the nut when installing on a fine threaded bolt.

How Deep Should I Thread a Fastener?

How deep a fastener needs to be threaded is directly related to its diameter. The bigger a fastener is, the deeper it will need to be in order to be strong. Many studies have found that there is a simple rule of thumb for standard pitch fasteners. The depth of a fastener should be at least two times its nominal diameter. For example a 1/4in (0.25in) fastener should be threaded at least 1/2in (0.50in) deep. Any deeper than this and the threads don’t really do much except add assembly time.Thread pitch also plays a part in how deep a fastener should be. For example, fine pitch threads distribute more loads per unit of length. Therefore they do not need to be threaded as deep. Most fine pitch threads obtain most of their strength by one diameter of engagement.

*Tip. When threading into thin material, use fine pitch threads when two diameters are not possible.


Written By: Sean Derrick – Product Development Engineer |
Sean has a B.S. in Engineering Design and a M.S. in Manufacturing Engineering from Western Michigan University. He has over 7 years of product development experience in a variety of industries ranging from medical, automotive, furniture, consumer products, and defense. Sean enjoys hiking, mountaineering, skiing, movies, and is pursuing a Ph.D in his free time.


17 thoughts on “Bolt Boot Camp: 6 Things to Know About Threaded Fasteners

  1. Number three was really interesting! I’m not an engineer by any means, but learning things like the difference between a bolt and a screw was pretty cool. I always wondered what the real difference was, but I guess now I know. Thanks for clarifying a lot of the confusion beginners like me face.

  2. Sean, good article — your distinction about when to use fine and coarse threads is important for engineers to remember! Thanks!

  3. How can I determine if bolt may be reused or must be replaced?
    For a particular trailer repair our techs may sometimes reuse a bolt if it looks to be in good shape or original condition. I’m not sure if that is a standard practice, or if bolts should simply be replaced by a new bolt each time? Thank you…Rick

    1. Hi Rick,
      In my own opinion the short answer is… If you wouldn’t want to try removing it after you re-install it then replace it. Most bolts and screws can be re-used if they are in good condition. If you have a bad feeling just remember replacing fasteners is cheaper than law suits and man hours. Long answer is… judge the condition of the bolt AS WELL as how tight it was installed. Condition is basic and pretty common sense. For example If the driver or head is getting stripped and the bolt will need to be remove a second time, don’t re-use it. You are just asking for trouble down the road. If the threads are damaged and re-installing the bolt will damage the threaded hole, don’t re-use it. If the bolt was rusted in place and difficult to remove…replace it. Corrosion, aesthetics, and age of the assembly all are condition related. So use your best judgement. However where you need to be careful has to do with how tight the bolts were installed. This is the unseen danger of judging bolt condition. Some bolts just can’t be re-used based on how they were installed. These are known as bolts which are “Torqued to Yield”. When they are installed the bolts are torqued to the point where the steel is stretched. Any more torque and the bolts will deform or even snap. When this is done on purpose its because the designer wanted the bolts to clamp as strongly as the bolts were capable of. But it can be done on accident if the installer used way too much torque. The amount of torque is based on the size of the fastener, the material its made of, and the thread size. The danger is if it suddenly takes an impact or is compromised in some way the bolt may snap. It will never be as strong as it was because it can’t rebound. But again…when in doubt…bolts are cheap. Hope this helps

  4. It’s great to know that these fasteners are able to hold and keep the material from sliding even in the strongest tension. I guess that is why my uncle badly needs it for his personal project. This is to ensure that it will not collapse when people use it once he has bought the right materials like this to construct it.

  5. It’s good to know that coarse thread fasteners are best when threading to the base material. My brother is working on building a new car, and he’s looking for a way to secure some things down. I’ll pass this information along to him so that he can look further into his options for fasteners and their carious thread types.

  6. Sean, Interesting and useful information. I am trying to ascertain the pullout strength of a #14-10×2″ screw versus a #14-14X 2″ screw and ran across this information. I would like to cite it to my employer that a 14-10 screw in 24 gauge metal is weaker than a 14-14 for pullout; I believe this to be correct from the information you have given. in #5. May I also ask your background or credentials?

  7. One of the fasteners on my trailer broke, and I just realized that it’s a U-bolt so I need to buy one if I plan to attach it to my pickup truck for my outdoor camping trip next month. It’s great to know that this type of fastener will need a nut in order to make sure that it’s secure enough to allow me to pull the trailer while I drive. I think I’ll look for someone who sells these in bulk in case I’ll need more of it or if I have to replace it.

  8. What a content..! Extremely useful for me ????.. Got the difference between nut and bolt and their pitches also

  9. I’m reconnecting two broken parts of a flat metal outdoor table leg using a separate piece of flat metal to re connect the two parts. I’m using 1/4-20 bolts. What is the minimum number of bolts needed for each of the two parts and their pattern placement?

  10. Thank you for sharing that the right nuts and bolts will ensure a secure fastener. My brother is undertaking a DIY project and is looking for nuts and bolts. I will advise him to ensure it is secured well.

  11. It’s helpful to know that that one of the most defining feature of a bolt compared to a screw is that it uses nuts. My newest project that I’d like to be able to finish by the end of this month is a bedframe that I will be using for my bedroom. I can see how being more knowledgeable about metric fasteners can ensure the durability of the entire structure.

  12. I asked my phone about deep threaded bolts. This is the article I got.
    Very technical yet I was able to understand every word.
    The mark of a good teacher.
    Thank you for the knowledge.

  13. I just drilled a hole in the crank shaft of a honda engine that came off a non=-working high pressure sprayer. I will replace the non workin engine on my lawn mower.with it. After I threaded the hole, I realized that I didn’t drill the hole for the bolt that holds the blade on, deep enough. I would have drilled the hole deeper and threaded it further, but I was afraid I would damage the threads.
    Your article that says twice the diameter of the bolt is enough. It is a fine thread, so I think I am good with a little less. Thank you very much for the great article

  14. It may be a good alternative to try the “v” angled thread files. The angle is 60% degrees so it is similar to the angle on Inch, Metric, Pipe, Male, and Female Threads. The files are basically round and they work great in restoring internal and external threads.

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