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—ready to start. But before you begin, there's one important question you might have overlooked: how deep should the thread go into the hole? The topic of thread length is often ignored, especially in DIY projects, but it's actually a crucial factor that affects the strength and reliability of your bolted connections.

Understanding how threads behave under stress and what influences their performance can be complex. While a mechanical engineering degree helps, you don’t need one to grasp the basics. A few simple rules are enough to ensure better stability and safety in most home or small-scale applications. After all, when it comes to securing parts together, extra safety never hurts.

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 always the same as the usable thread length you end up with. This distinction is key, especially when working with different types of holes.

For a through hole, the thread length is typically limited by the thickness of the material. You can’t cut a thread where there’s no material to support it. So, in many cases, the effective thread length will be shorter than the maximum the tap could produce.

Things get trickier with a blind hole, which ends inside the material. Since the tap can’t exit on the other side, you must choose the thread depth carefully. Here, the usable thread length becomes even more important. It’s calculated by subtracting the tap’s chamfer from the total thread length. These areas aren’t fully formed and don’t contribute to the actual threaded section. That’s why, for blind holes, the usable thread length is always less than the total thread length.

How Screw-In Depth Relates to Thread Length

You might be wondering why this matters. Well, the screw-in depth—how far the screw goes into the tapped hole—is directly influenced by the thread length. This depth determines how much of the screw’s threads overlap with those of the nut or internal thread, which is essential for a strong connection.

Screw-in depth is usually expressed as a multiple of the screw diameter (e.g., 1.5d means 1.5 times the diameter). This value ensures that the load is spread evenly across the threads, preventing them from stripping out. Even though some people still believe only the first three threads carry the load, in reality, all threads share the stress to some extent. That’s why having enough threads is so important.

If the thread length is too short, the screw may not engage properly, leading to a weak or unreliable joint. In critical applications, this can be dangerous. So getting the thread length right is not just about function—it’s about safety.

What Happens if the Thread Length Is Wrong?

Determining the correct thread length involves several factors, such as the material strength, screw diameter, thread type, and the intended use. For example, high-load applications require longer thread lengths to ensure durability and prevent failure.

But here’s a common misconception: more thread length isn’t always better. In fact, excessively long threads can be hard to produce and may cause issues during tapping. They can also make assembly harder and increase the risk of jamming or seizing.

So the golden rule is: as much as needed, as little as possible. Finding the right balance ensures both efficiency and reliability in your fastening system.

How to Find the Right Thread Length

There are many resources online to help you determine the appropriate thread length. Screw-in depth charts and online calculators can give you a good starting point. Just make sure to account for the usable thread length of your tap, especially if you're working with blind holes.

Also, be cautious if the recommended values seem unusually high. Avoid thread lengths greater than three times the screw diameter. In such cases, consider using materials that provide better thread engagement with shorter lengths.

Once you’ve determined the right thread length, take a look at our BAER online store to find the perfect tap for your project. If you have any questions, we’re here to help. Safe and secure fastening starts with the right tools—and the right knowledge.