When you need a bolt, it’s tempting to find whatever’s around that meets the size specifications you need. But so much more goes into choosing the right solution for the job. There are considerations like how well a bolt grips and how it distributes a load. There are many types of bolts, screws and other fasteners to support different jobs with varying requirements.
When you have a specific job at hand and need dependable screws, you have many decisions to make. One of the most important choices is whether you need a partially or fully threaded bolt. To help you gain more insight and make the best decision for any project, here’s a look at full thread vs. partial thread screws.
We have all been guilty of the “if it fits, it is close enough,” mentality, but it’s not the best practice. When you rely on the experts at FMW Fasteners, we can help you find the bolts and screws that meet your requirements.
Screw vs. Bolt
Used as an umbrella term for both screws and bolts, a fastener is a hardware device that mechanically holds two or more applications in a definite position. A fastener creates a non-permanent joint that you can tighten or remove when necessary.
The global fasteners market size was estimated at USD 83.3 billion in 2018 and is expected to expand at a compound annual growth rate of about 4% by 2025. In 2018, automotive applications that used fasteners took the lead in North America by accounting for about 29% of the overall fasteners market. Other industries that use fasteners include industrial machinery, aerospace, building and construction, home appliances, furniture, motors and pumps, lawn, plumbing, DIYs and more.
So, what is the difference between a screw and a bolt? A screw is an externally threaded fastener. It has a head at one end that you can turn to tighten, and the other end has a helical thread to pierce through surfaces. Screws are matched with a pre-formed internal thread of a hole, or the fastener forms its own thread. You can tighten and release it by turning the head.
A bolt is also an externally threaded fastener with a head on one end. However, the difference lies in how it applies. Bolts insert through pre-drilled holes. In conjunction with a nut and sometimes a washer, you can then tighten the fastener. The entire piece comprises of a head, flange, body, transition and thread.
What Is a Fastener Thread?
A fastener thread is a ridge that’s wrapped around the cylinder of a bolt, screw or another fastener. Known as a parallel or straight thread, the uniform section forms a helix that can be on the internal or external surface of a cylinder. There is also a difference between a right- and left-handed thread.
A right-hand thread will wind clockwise in receding direction when you view it axially. Most threads hold this design. A left-hand thread, when looked at axially, has threads that turn counterclockwise.
How Many Types of Threads Are There?
Two main types of threads exist — which you already know — partial and full. However, there are also various kinds of threads found on either a full or partial fastener such as:
- Unified National Coarse: UNC threads are for general fastening jobs that have a deeper and more generic fit for easy removal.
- Unified National Extra Fine: UNEFs are the finest types of threads and are ideal for applications that deal with hard material with tapped holes, thin threaded walls and thin material with tapped holes.
- Unified National Fine: UNF threads have increased load-carrying and torque-locking capabilities thanks to their big minor diameter. They have tighter tolerances, can withstand heavier loads and have finer tension adjustment.
- UNJC and UNJF: While there are internal and external “J” threads, external UNJC and UNJFs have a bigger root radius, giving the thread larger tensile strength and less stress concentration.
- UNR and UNK: A UNR is the same as a UNC, but it has a rounded root radius and is only external. A UNK thread is like a UNR, but the minor widths and root radius need inspections.
- Constant-Pitch: Constant-pitch threads are available in various diameters to fit specific applications.
On top of operating with a variety of different threads for your project, you can also pick from an array of bolts like carriage, hex, lag, flange and eye. Or, choose from screws like wood, machine, sheet metal and mating. Each can have a range of other characteristics like head style, drive types, washers and nut types. With the support of our expertise, we can help you narrow down your options.
Thread Pitch vs. Thread Diameter
The same method continues when you’re searching for the correct bolt for your application. Do you know how to determine thread pitch, and do you know what thread diameter is?
To start, thread count is measured by the threads per inch (TPI) of a fastener. It describes the number of threads per inch along the length of a bolt or screw and is a term used for American fasteners that use the imperial system. When you’re working with a smaller fastener, it will have smaller threads, meaning the thread count will be higher — and vice versa.
Screw pitch uses metric units to measure the distance between threads. Measured in millimeters, it is the distance from one point on the thread to another corresponding point on the adjacent thread, which is measured parallel to the axis in the same axial plane and on the same side. To measure both, use a thread gauge or rely on the specialists at FMW Fasteners to tell you what you need.
You should also familiarize yourself with the pitch diameter which is the diameter of the thread that passes through the thread. The widths of the ridges, as well as the thread grooves, are equal. The pitch diameter is about halfway between the minor and major diameters.
There is also the case of knowing the difference between major and minor diameter. Major is a measurement that calculates the distance from crest to crest of an external thread. For an internal thread, you would go from root to root of the imaginary cylinder. Minor is the diameter that touches the crests or roots of the imaginary cylinder that’s also known as the root diameter.
Partial Thread Fasteners
Finally, down to the most essential phase of knowledge. Partial thread bolts and screws offer an area below the head that is entirely free of threading. They are called partial thread bolts because of the part that has no threading. This non-thread area of a bolt, also known as the bolt shank or shoulder of the fastener, will vary in length depending on the bolt's application.
Why choose a partial thread bolt? Partial thread bolts are best for resistance and alignment. When you have a project that requires a lot of force to hold an object in place, a partially threaded bolt delivers the strength needed. The non-threaded segment, known as grip length, contains zero weak spots to prevent strain.
When using partially threaded bolts, you can drive them into threaded holes, or you can use them for a bolt-through application along with a nut and washer. Partial threads are ideal for additional strength, shear resistance and critical alignment as opposed to better grip. Use them on applications like motor mounts, water pumps, alternators and more.
Full Thread Fasteners
As you might have guessed, fully threaded bolts and screws have no grip length or thread-free portion. The threads run from under the head to the tip.
Why choose a fully threaded bolt? If you’re most interested in grip strength and less interested in alignment and shear strength, a fully threaded bolt is going to be your best option. Once mounted, a full thread bolt spreads pressure along its entire length, with the largest holding pressure placed on the head where it meets the material it fastens to.
Often used in the automobile industry to design vehicle frames, full thread solutions distribute the mounting pressure of a car’s panels across the length of the entire bolt thanks to the full threading. Where grip strength is more critical compared to alignment or shear strength, full threads are your go-to.
What’s the Difference Between the Two Options?
The major difference of fully threaded screws vs. partially threaded screws is that full fasteners have threads that run the whole length of the screw, while partial screws have an unthreaded section below the head, which is the grip length. Then there is partial threading the rest of the way down. Both types of fasteners are ideal for a variety of applications, but it’s crucial to know which fits your project as they grip and distribute weight differently. Variables like the head size, thread pitch and type of thread all affect a fastener’s capabilities.
How You Can Use Various Fastener Types
No one type of bolt is used more than another. Instead, the bolt you select is all about the fastening you need.
Choose partially threaded bolts for projects that demand extreme holding force. Partial thread bolts are often used for motor mounts, alternators and other automotive fastenings, as well as water pumps. In these types of applications, precision is vital and the grip length of the bolt delivers the alignment that’s needed.
During tightening, the grip length is pulled tight into the hole, yanked in by the threads that are exposed on the other side of the hold. The resulting force is what’s needed to hold motor mounts securely. Partial threads are also better at resisting cracks and bends.
Choose fully threaded bolts for applications that need more grip strength than holding force. If you would use partially threaded bolts for automotive fastenings, like motor mounts and alternators, use full thread bolts for automotive fastening like body panels. Body panels do not require the same amount of holding force that motor mounts demand, but they do need a stable grip. A partially threaded bolt stores its holding force at the threaded end, while a fully threaded bolt's holding force expands across the entire shaft section.
Working with a fully threaded fastener means the tension placed on the bolt will spread over a larger surface area to withstand larger forces. They are also less likely to come loose.
Choosing Between Fully Threaded Bolts and Partially Threaded Bolts
You face many choices and options when searching for the right bolt for your next project. When it comes to choosing full thread vs. partial thread screws, consider whether you need maximum grip strength or maximum protection against shear. If you need grip strength, find the right full thread bolt that’s made of the material you need. If you’re more concerned about shear, choose a partially threaded bolt that’s going to help protect against it.
You’ll find that different partially threaded bolts have varying grip lengths, so consider how long you need a grip length to be before making your purchase. If you are still unsure of which solution will best apply to your job, rely on the experts from FMW Fasteners for guidance.
About FMW Fasteners
At FMW Fasteners, we serve both professionals and do-it-yourself enthusiasts who need high-quality materials to create outstanding results with their projects. You’ll find a vast selection of both fully and partially threaded bolts, as well as all the other fasteners you might need for the job at hand. You can order in the exact amount you need rather than settling for a random bulk quantity.
Enjoy everyday low pricing at FMW Fasteners, as well as free shipping on qualified orders. You can also send back what you don’t want or need at any time.
Have questions about full thread vs. partial head screws? We're always here to help. Not only will you find the best selection of materials at FMW Fasteners, but you'll also find a customer service team that's focused on helping you find what you need. Everything we do — from our selection and ordering process to our customer service — exceeds the needs of professionals and DIY enthusiasts who want the best materials to deliver exceptional results.
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