When shopping for piercing jewelry or replacement tops, you'll often come across terms such as Internal Thread and External Thread. If you're not familiar with these systems, the difference can be confusing at first.

In reality, they are simply two opposite threading systems.

In an internal thread system, the screw is located on the back of the decorative top, while the threads are located inside the bar. To assemble the jewelry, the screw on the decorative top is inserted and tightened into the bar.

In an external thread system, the structure is reversed. The screw is located on the end of the bar, while the threads are inside the decorative top. The bar is then screwed into the top to secure the jewelry.

Why Is It Called an Internal Thread Bar and Not an External Thread Top?

This is one of the most common sources of confusion.

In the piercing jewelry industry, threading systems are generally classified based on the bar rather than the decorative top.

For this reason, even though a decorative top designed for an internal thread system has a screw on its back, it is not considered an "external thread top."

Instead, it is described as a decorative top designed for an internal thread bar.

This naming convention is widely used throughout the industry and helps avoid confusion when identifying compatible components.

Why Doesn't a 1.2 mm Bar Use a 1.2 mm Screw?

Another common misunderstanding concerns sizing.

When a product is labeled as 1.2 mm, many people assume that every component measures exactly 1.2 mm. However, that is not the case.

For a screw to fit inside a bar, it must be smaller than the outer diameter of the bar itself.

For example, a 1.2 mm internal thread bar will typically use a screw measuring 0.9 mm or 1.0 mm. The screw needs to be smaller than the bar so that it can be inserted and threaded correctly.

A 1.2 mm screw simply would not fit inside a 1.2 mm bar.

Why Aren't Decorative Tops Usually Sold as 0.9 mm or 1.0 mm?

Although the screw on a decorative top may measure 0.9 mm or 1.0 mm, most manufacturers do not use the screw size as the primary specification when selling jewelry.

Instead, products are usually labeled as:

"Fits 1.2 mm Internal Thread Bars"

or

"Compatible with 1.2 mm Internal Thread Jewelry"

This approach is much easier for customers because they only need to know the size of their bar rather than the exact dimensions of the screw.

It also helps reduce confusion, since thread specifications can vary slightly between manufacturers.

What Should You Check Before Buying a Replacement Top?

Before purchasing a replacement top, it's important to identify which threading system your jewelry uses.

First, determine whether your jewelry is internally threaded or externally threaded. Next, check the size of your bar and make sure the replacement top is designed for that system.

Taking a few minutes to verify these details can help avoid compatibility issues and make choosing the correct jewelry much easier.