# How to Identify an Unknown Thread Type

Figure 1: Several thread standard profiles

## Parameters for identifying a thread

There are four factors to determine when identifying a thread:

1. External or internal threads
2. Straight or tapered threads
3. Thread nominal diameter

### External vs internal thread

External threads are male threads, and internal threads are female threads. To identify a thread type, measure the major diameter:

• External thread major diameter: The major diameter is measured from crest to crest (high point to high point) across the thread axis, an imaginary line running through the center of the thread.
• Internal thread major diameter: The major diameter is measured from root to root (low point to low point) across the thread axis.

Figure 2: Thread dimensions: pitch (A), flank angle (B), minor diameter (C), pitch diameter (D), major diameter (E), depth (F), crest (G), and groove (H)

A straight thread has a constant nominal diameter across its length, whereas a tapered thread does not. Measure the nominal diameter of a straight thread at any point. For a tapered thread, measure the nominal diameter at the 4th or 5th thread from the head to get the true measurement.

Figure 3: A straight male thread with constant major diameter (left) and a tapered male thread with varying major diameter (right)

### Nominal diameter and pitch

As discussed above, the nominal diameter of an external thread is the distance from crest-to-crest across the thread axis. And the nominal diameter of internal threads is the distance from root-to-root across the thread axis. To precisely measure the diameter of any thread, use a Vernier caliper.

A thread’s pitch is the distance between two consecutive crests. The metric thread standard uses this measurement in its designation. For example, M10 x 1.25. 1.25 is the pitch in millimeters. UTS threads use TPI (thread-per-inch) in their designation, which is the inverse of pitch. For example, #6 - 32 UNC. The 32 is the thread-per-inch. Read our guide on how to measure thread size to learn more.

After identifying a thread’s parameters, look in thread standard charts to compare the thread’s nominal diameter and pitch. Start by checking our thread standards overview article and individual thread standard articles:

• UTS
• Metric
• NPT (National Pipe Thread)
• BSP (British Standard Pipe)

### Example

An external screw thread with approximately 0.5 mm pitch and 3.0 mm major diameter. This example shows why a precise caliper is necessary when identifying a thread. Some metric threads are very similar to UTS threads. A M3 x 0.5 thread has a 2.980 mm major diameter and 0.5 mm pitch. A unified fine thread has a 0.53 mm pitch and a 2.84 mm major diameter. In this example, the major diameter is very close to 3.0 mm. Therefore, the screw thread is metric and not UTS.