Everything you need to know about thread length

Imagine you've finally found the perfect tap for your cutting job, prepared the workpiece, and carefully set up your machine. You're all set to start—right? But wait, there's one key detail that might still be unclear: How deep should the thread go into the hole? The topic of thread length is often overlooked, especially in DIY projects, but it’s not something you can ignore if you want secure and long-lasting connections.

Understanding how threads work, what forces act on them, and how this affects the required thread length can be complex. While a degree in mechanical engineering would help, you don’t need one to grasp the basics. A few simple principles are enough for most home users and will add an extra layer of stability and safety to your bolted joints. After all, when it comes to strength, more is always better—especially when it matters.

Thread Length vs. Usable Thread Length – What’s the Difference?

The part of the tap where the actual threads are cut is called the thread length. However, this isn’t the same as the full usable thread you can create. There’s an important distinction here that you should know about.

Let’s start with a simple case: cutting a thread in a through hole. In most cases, the thread length is limited by the thickness of the material. You can’t cut a thread where there’s no material to begin with. So, for through holes in sheet metal or plates, the actual thread length will usually be less than the maximum the tap can produce.

Things get trickier with a blind hole, which ends inside the material. Since the tap can't exit on the other side, you have to decide how deep you want the thread to go—and this also determines the depth of the core hole. Here, the difference between thread length and usable thread length becomes critical. The usable thread length is always shorter because it excludes the chamfer at the end of the tap. These areas aren’t fully threaded and don’t count toward the final thread length. For blind holes, the usable thread length is simply the thread length minus the chamfer.

How Screw-In Depth Relates to Thread Length

You might be wondering why this matters. Well, the thread length directly affects the screw-in depth, which is the distance the screw goes into the nut or threaded hole. This is a crucial factor in ensuring a strong, secure connection.

The screw-in depth refers to the area where the threads of the screw and nut overlap. It’s typically expressed as a multiple of the screw’s diameter. A screw-in depth of 1.5 times the diameter means the screw is inserted 1.5x deeper than its width. This ensures that the load is evenly distributed across the threads, preventing them from tearing out under stress.

Even though some people think only the first few threads carry the load, in reality, all threads share the burden. That’s why it’s essential to have enough threads to handle the force. If the thread length is too short, the connection may not be strong enough, especially in high-stress situations.

What Happens if the Thread Length Is Wrong?

So, what’s the right thread length? It depends on several factors, including the strength grade of the fastener, the thread diameter, the material’s shear strength, and the intended use. The more load the connection has to bear, the more important the thread length becomes.

You might think longer is always better, but that’s not true. Threads that are too long can be hard to cut, increase the risk of jamming or seizing, and make assembly more complicated. Plus, they don’t necessarily provide more strength—they just add unnecessary complexity.

The golden rule is: As much as needed, as little as possible. This approach keeps things efficient, safe, and easy to manage.

How to Determine the Right Thread Length

If you’re unsure, you can find screw-in depth charts online or use thread length calculators that take your materials and dimensions into account. Just remember to check the usable thread length of your tap, especially when working with blind holes.

Also, watch out for unusually high values in these tools. If the recommended thread length is more than three times the diameter, it might be worth considering a different material or design that offers better performance with shorter threads.

Once you’ve determined the correct thread length, visit our BAER online store to find the ideal tap for your project. If you have any questions or run into issues, we’re here to help—because safety and quality should never be compromised.