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When it comes to purchasing threaded components such as bolts, nuts, or pipes, even the replace quick couplings, getting the correct thread size is crucial. An incorrect size can lead to fitting issues and potential damage. In this guide, we’ll walk you through the steps to accurately measure thread size, ensuring you buy the right item every time.
Step-by-Step Guide to Measuring Thread Size
Gather Your Tools
To measure thread size accurately, you’ll need the following tools:
- A caliper or a ruler for measuring diameter
- A thread pitch gauge for measuring thread pitch
- A reference chart for thread size identification
Measure the Outer Diameter (OD)
For male threads (external threads like bolts), measure the outer diameter using a caliper. Place the caliper jaws on the outer edges of the thread and note the measurement. For female threads (internal threads like nuts), measure the inner diameter.
Tip: Ensure the caliper is perpendicular to the thread axis for an accurate measurement.
Determine the Thread Pitch
Thread pitch is the distance between threads. Use a thread pitch gauge to measure this. Match the gauge’s teeth to the threads of your item. The correct gauge will fit snugly into the threads without any gaps.
- Metric Threads: Measure the distance between adjacent threads in millimeters (e.g., 1.5 mm).
- Imperial Threads: Measure the number of threads per inch (TPI) (e.g., 16 TPI).
Identify the Thread Type
Threads can be classified as coarse or fine. Coarse threads have fewer threads per inch (or a larger pitch in metric) compared to fine threads.
Example:For a bolt with an outer diameter of 10 mm and a pitch of 1.5 mm, it’s an M10x1.5 bolt (M10 denotes the diameter, and 1.5 denotes the pitch).
Consult a Thread Size Chart
With the measurements in hand, refer to a thread size chart to identify the exact thread size. These charts list the diameter, pitch, and corresponding thread size designation.
How to Measure NPT Thread:
NPT (National Pipe Thread) is a U.S. standard for tapered threads used on pipes and fittings. Accurately measuring NPT threads is essential to ensure compatibility and prevent leaks in plumbing, gas lines, and other threaded applications. This guide will walk you through the process of measuring NPT threads effectively.
Step-by-Step Guide to Measuring NPT Threads
Gather Your Tools
A caliper for measuring diameters
- A thread pitch gauge for measuring thread pitch
- A reference chart for NPT thread sizes
- reference chart for NPT thread sizes
Measure the Outside Diameter (OD) for Male Threads
For male (external) NPT threads, use a caliper to measure the outside diameter. Take the measurement at the widest point of the thread.
Tip: Ensure the caliper is perpendicular to the axis of the thread to get an accurate measurement.
Measure the Inside Diameter (ID) for Female Threads
For female (internal) NPT threads, measure the inside diameter at the largest point of the thread. This can be slightly more challenging due to the taper.
Determine the Thread Pitch
Use a thread pitch gauge to find the pitch of the threads. The pitch is the distance between threads. NPT threads typically follow a standard pitch for each size.
Example: For a 1/2 inch NPT thread, the pitch is usually 14 threads per inch (TPI).
Measure the Taper Angle
NPT threads are tapered, not straight. This taper is 1° 47′ (1.78°) over the length of the thread. This taper ensures a tight fit when the threads are engaged.
Consult an NPT Thread Size Chart
With the measurements in hand, use an NPT thread size chart to identify the correct size. NPT threads are designated by their nominal pipe size, which doesn’t directly correspond to the actual diameter measurements.
| Nominal pipe size |
Thread density |
Thread pitch P |
Length L1 |
Diameter E1 |
Length L2 |
Diameter E2 |
Overall length L4 |
Actual outside diameter D |
Tap drill |
|---|---|---|---|---|---|---|---|---|---|
| 1⁄16 | 27 | 0.03703704 | 0.16 | 0.28118 | 0.2611 | 0.2875 | 0.3896 | 0.313 | |
| 1⁄8 | 27 | 0.03703704 | 0.1615 | 0.3736 | 0.2639 | 0.38 | 0.3924 | 0.405 | 0.339 |
| 1⁄4 | 18 | 0.05555555 | 0.2278 | 0.49163 | 0.4018 | 0.5025 | 0.5946 | 0.54 | 7⁄16 |
| 3⁄8 | 18 | 0.05555555 | 0.24 | 0.62701 | 0.4078 | 0.6375 | 0.6006 | 0.675 | 37⁄64 |
| 1⁄2 | 14 | 0.07142857 | 0.32 | 0.77843 | 0.5337 | 0.79178 | 0.7815 | 0.84 | 23⁄32 |
| 3⁄4 | 14 | 0.07142857 | 0.339 | 0.98887 | 0.5457 | 1.00178 | 0.7935 | 1.05 | 59⁄64 |
| 1 | 11 1⁄2 | 0.08695652 | 0.4 | 1.23863 | 0.6828 | 1.25631 | 0.9845 | 1.315 | 1 5⁄32 |
| 1 1⁄4 | 11 1⁄2 | 0.08695652 | 0.42 | 1.58338 | 0.7068 | 1.60131 | 1.0085 | 1.66 | 1 1⁄2 |
| 1 1⁄2 | 11 1⁄2 | 0.08695652 | 0.42 | 1.82234 | 0.7235 | 1.84131 | 1.0252 | 1.9 | 1 47⁄64 |
| 2 | 11 1⁄2 | 0.08695652 | 0.436 | 2.29627 | 0.7565 | 2.3163 | 1.0582 | 2.375 | 2 7⁄32 |
| 2 1⁄2 | 8 | 0.125 | 0.682 | 2.76216 | 1.1375 | 2.79063 | 1.5712 | 2.875 | 2 5⁄8 |
| 3 | 8 | 0.125 | 0.766 | 3.3885 | 1.2 | 3.41563 | 1.6337 | 3.5 | 3 1⁄4 |
| 3 1⁄2 | 8 | 0.125 | 0.821 | 3.88881 | 1.25 | 3.91563 | 1.6837 | 4 | 3 3⁄4 |
| 4 | 8 | 0.125 | 0.844 | 4.38713 | 1.3 | 4.41563 | 1.7337 | 4.5 | 4 1⁄4 |
| 4 1⁄2 | 8 | 0.125 | 5 | 4 3⁄4 | |||||
| 5 | 8 | 0.125 | 0.937 | 5.44929 | 1.4063 | 5.47863 | 1.84 | 5.563 | 5 9⁄32 |
How to Measure BSP Thread
British Standard Pipe (BSP) threads are used extensively for pipes and fittings, especially in Europe and the Commonwealth countries. Measuring BSP threads accurately ensures compatibility and prevents leaks in plumbing and other threaded applications. This guide will walk you through the process of measuring BSP threads effectively.
Step-by-Step Guide to Measuring BSP Threads
Gather Your Tools
To measure BSP threads accurately, you will need:
- A caliper or a ruler for measuring diameter
- A thread pitch gauge for measuring thread pitch
- A reference chart for BSP thread sizes
Measure the Outside Diameter (OD) for Male Threads
For male (external) BSP threads, use a caliper to measure the outside diameter. Place the caliper jaws on the outer edges of the thread and note the measurement.
Tip: Ensure the caliper is perpendicular to the thread axis for an accurate measurement.
Measure the Inside Diameter (ID) for Female Threads
For female (internal) BSP threads, measure the inside diameter at the largest point of the thread. This can be slightly more challenging due to the taper (in BSPT) or parallel (in BSPP) nature of the threads.
Determine the Thread Pitch
Use a thread pitch gauge to measure the pitch of the threads. The pitch is the distance between threads. Match the gauge’s teeth to the threads of your item. The correct gauge will fit snugly into the threads without any gaps.
Example: For a BSP thread with a pitch of 1.5 mm, use the corresponding pitch gauge.
Identify the Thread Type: BSPT vs. BSPP
BSP threads come in two types:
- BSPT (British Standard Pipe Taper): Tapered threads that provide a tight seal.
- BSPP (British Standard Pipe Parallel): Parallel threads that require a sealing washer or O-ring.
Tip: Visually inspect the threads to determine if they are tapered (BSPT) or parallel (BSPP).
Consult a BSP Thread Size Chart
With the measurements in hand, use a BSP thread size chart to identify the correct size. These charts list the diameter, pitch, and corresponding BSP size designation.
Tips for Accurate Measurement
- Clean the threads before measuring to remove any debris or corrosion.
- Ensure the thread pitch gauge fits snugly into the threads without any gaps.
- Double-check your measurements for consistency and accuracy.
- Remember that BSPT threads are tapered, while BSPP threads are parallel.
Accurately measuring BSP threads is essential for ensuring a proper fit and preventing leaks in threaded pipe systems. By following this guide and using the correct tools, you can confidently determine the size of BSP threads. If you have any questions or need further assistance, please contact our customer support team.
