Seven Thread Types: Side-by-Side Comparison

[Abstract]:In the field of fasteners and mechanical connections, seven widely used thread types including NPT, PT, G, TR, S, ZG and RC are frequently confused in practical application.

In the field of fasteners and mechanical connections, seven widely used thread types including NPT, PT, G, TR, S, ZG and RC are frequently confused in practical application. Clarifying their core differences is essential for accurate thread selection and reliable connection performance. This article systematically analyzes the seven thread types from the dimensions of technical standards, structural features, sealing characteristics and application scenarios.

These threads can be categorized by functions into three groups: sealed pipe threads, non-sealed fastening threads and transmission threads. The sealed pipe thread group includes NPT, PT, ZG and RC, which are mainly applied for pressure fluid pipeline sealing. G and S threads belong to non-sealed threads for positioning and mechanical fixing. TR threads are dedicated transmission threads for force and motion transfer.

Among sealed pipe threads, NPT is a US tapered pipe thread complying with ANSI B1.20.1 standard, featuring a 60° thread angle and a 1:16 taper ratio. It achieves sealing through thread deformation and requires sealant during assembly. NPT threads are widely used in high-pressure hydraulic and pneumatic pipelines, such as hydraulic connectors for construction machinery. PT (BSPT) is a British tapered pipe thread conforming to ISO 7/1 standard with a 55° thread angle and the same 1:16 taper, realizing sealing through precise thread surface fitting and commonly adopted for water and air pipe connections on European equipment.

ZG and RC both represent British tapered pipe threads. ZG is an old national standard marking, corresponding to the new standard RC tapered internal thread under GB/T 7306.2, with a 55° thread angle and 1:16 taper. RC threads must be matched with tapered external threads (RP for cylindrical internal threads) to form effective sealing, and are mostly used for low-pressure sealing scenarios such as heating, ventilation and water supply systems, including radiator interfaces. It is noteworthy that PT and RC threads follow different standard systems and cannot be interchanged despite both being British tapered pipe threads.

For non-sealed threads, G thread, also known as BSPP, is a British standard non-sealed cylindrical pipe thread complying with ISO 228-1 standard with a 55° thread angle. It has no self-sealing capability and relies on gaskets or sealant for leakage prevention, suitable for low-pressure pipelines and ventilation ports such as equipment fuel tank vents. S thread refers to metric standard cylindrical threads under GB/T 196 with a 60° thread angle. As universal fastening threads applied for ordinary bolts and nuts, S threads are mainly used for fixed connection of mechanical components.

TR threads are trapezoidal threads implemented under GB/T 5796, featuring a 30° thread angle and isosceles trapezoid profile. Different from fastening and sealing threads, TR threads prioritize transmission efficiency and load-bearing capacity. Their large force-bearing tooth area reduces wear and makes them ideal for transmission mechanisms such as machine tool feed lead screws and elevator drive screws.

The seven threads also differ significantly in assembly modes. NPT, PT, ZG and RC are tapered threads that enhance sealing and fastening performance through tapered surface fitting during installation. G, S and TR are straight threads that require nuts or positioning structures for fixation. In terms of tooth profile, 60° threads (NPT, S, TR) focus on load bearing and motion transmission, while 55° threads (PT, G, ZG, RC) adapt better to sealing requirements of British-standard equipment.

Their application scenarios are clearly differentiated. NPT threads are preferred for high-pressure fluid systems; PT threads for low-pressure sealing of European equipment; RC (ZG) threads for heating and ventilation systems; G threads for low-pressure non-sealed pipelines; S threads for general component fastening; and TR threads for mechanical transmission mechanisms. For instance, automotive brake pipelines adopt NPT threads for reliable high-pressure sealing, while desk lifting lead screws adopt TR threads for stable power transmission.

Fastener practitioners must focus on the standard compatibility and sealing adaptability of different threads. NPT and PT threads are non-interchangeable due to different thread angles and standard systems. RC internal threads must be matched with corresponding tapered external threads; mismatching with G threads will directly cause medium leakage. Accurate thread selection effectively improves product reliability and reduces after-sales failure risks.

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Seven Thread Types: Side-by-Side Comparison

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