At RUIHUA HARDWARE, we solve hydraulic connection puzzles every day. One of the most common—and critical—questions we encounter is the fundamental difference between metric and imperial threads. Choosing incorrectly can lead to leaks, system failure, and costly damage. This guide will clarify the confusion and ensure you make the perfect connection.
The Core Difference: It’s More Than Just Inches vs. Millimeters
While the measurement systems differ, the key distinction lies in their sealing philosophy.
Metric Threads (e.g., M, MZ): The thread itself is NOT the seal. Parallel metric threads (like M12x1.5) rely on a separate sealing element, such as an O-ring or a metal washer, compressed against a face. This design offers excellent, reliable sealing and is the modern international standard (ISO).
Imperial Threads (e.g., BSPP, BSPT, NPT): The thread is often PART OF the seal. Tapered threads (like BSPT or NPT) create a seal through the metal-to-metal interference of the threads themselves, typically requiring sealant tape or paste. Parallel imperial threads (BSPP/G-series) use a bonded washer for sealing.
Quick-Reference Comparison Table
Feature
Metric Thread (ISO)
Imperial Thread (BSPP)
Imperial Thread (NPT)
Standard
ISO 6149, ISO 1179-1
ISO 228-1 (BSPP)
ANSI/ASME B1.20.1
Thread Angle
60°
55°
60°
Common Types
Parallel (M), Tapered (MZ)
Parallel (G), Tapered (R)
Tapered (NPT/NPTF)
Pitch Notation
Millimeter distance between threads (e.g., 1.5mm).
Threads Per Inch - TPI (e.g., 14 TPI).
Threads Per Inch - TPI (e.g., 14 TPI).
Size Label
Nominal major diameter (e.g., M12x1.5).
Nominal bore size, not actual thread size (e.g., G1/4").
Nominal bore size, not actual thread size (e.g., 1/4" NPT).
Sealing Method
O-ring, washer, or metal cone face.
Sealing washer on the fitting face.
Tapered thread engagement (requires sealant).
Primary Region
Global, especially Europe & Asia.
Europe, legacy equipment worldwide.
North America.
The RUIHUA HARDWARE Expert Guide to Identification & Selection
1. The Golden Rule: NEVER Force It! If threads don’t screw together smoothly by hand, STOP. Forcing a "close-enough" fit will cross-thread and ruin the connection. A fitting that seems to start but feels tight is a major red flag. 2. How to Identify What You Have:
Measure the Outer Diameter: Use a caliper on the male thread.
Determine the Pitch/TPI: This is the most crucial step. Use a thread pitch gauge.
Observe the Thread Angle: 60° points to Metric or NPT; 55° points to BSP.
Check for Taper: Place the fitting against a straight edge. Tapered threads are obvious.
3. Our Recommendation for Modern Systems:
As a leading manufacturer, RUIHUA HARDWARE champions metric-threaded fittings with O-ring seals (e.g., to ISO 1179-1). They provide leak-free reliability, withstand vibration, and allow for multiple reassemblies without degradation—making them the superior choice for new designs and maintenance. 4. Solving the Conversion Challenge: Have an imperial port but need to connect a metric hose? Do not improvise. The safe solution is to use a properly engineered transition adapter. RUIHUA manufactures and stocks a wide range of these adapters to bridge any thread standard safely and reliably.
Partner with Precision, Avoid Costly Downtime
Understanding thread types is essential for system integrity. At RUIHUA HARDWARE, we don’t just supply fittings; we provide the expertise to ensure your hydraulic systems are built on secure, leak-free foundations.
Struggling with a thread identification challenge?Ready to specify the reliable, modern metric standard for your project?Contact RUIHUA HARDWARE today. Let our technical team be your guide to flawless hydraulic connections.
