Are metric and inch threads interchangeable?

No. Metric and inch threads are NOT interchangeable, even when the diameters are similar. The thread pitch and flank angle differ. Metric threads use a 60° angle with pitch in mm. UNC/UNF also use 60° but with TPI (threads per inch). BSW uses a 55° angle. Forcing a wrong thread will damage both bolt and nut.

How to identify thread type?

Use a thread pitch gauge to measure the pitch. Measure the major diameter with calipers. If pitch is in whole mm values (0.5, 0.7, 1.0, 1.25, 1.5, 2.0, etc.), it's likely metric. Count threads per inch for imperial. Check if the thread angle is 60° (metric/unified) or 55° (BSW/BSP).

What is the difference between pitch and TPI?

Pitch is the distance between adjacent thread crests in millimeters (used for metric threads). TPI (Threads Per Inch) is the number of threads in one inch (used for imperial threads). They are inversely related: Pitch (mm) = 25.4 / TPI.

BSW (British Standard Whitworth) is a thread standard developed by Sir Joseph Whitworth in 1841. It uses a 55° thread angle (vs 60° for metric and unified). BSW has largely been replaced by metric and unified standards but is still found in legacy equipment, plumbing (BSP), and some British machinery.

What is the difference between UNC and UNF?

UNC (Unified National Coarse) has fewer threads per inch — it's the standard/default thread. UNF (Unified National Fine) has more threads per inch for the same diameter. UNF provides: better vibration resistance, higher tensile strength per diameter, finer adjustment capability. UNC is preferred for: general assembly, tapped holes in softer materials, and ease of assembly.

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螺紋尺寸轉換器

公制 (ISO)、UNC、UNF 與 BSW 螺紋尺寸之交叉比對。輸入任意螺紋規格或選取公制尺寸，即可查詢最接近的等效規格。

公制 ISO UNC / UNF BSW Whitworth

螺紋交叉比對

公制	Ø mm	螺距 mm	UNC	UNF	BSW	應力面積 mm²

螺紋標準總覽

全球通用的四大主要螺紋標準：

公制 (ISO 261/262) — 標示為 M + 直徑 (例如 M8)。螺距單位為 mm。螺紋角度 60°。全球最常見。
UNC (Unified National Coarse) — 直徑-TPI 格式 (例如 5/16-18)。角度 60°。美國/加拿大標準粗牙螺距。
UNF (Unified National Fine) — 與 UNC 相同系統，但螺距更細 (TPI 更高)。抗振動性更佳。
BSW (British Standard Whitworth) — 螺紋角度 55°。直徑以英吋表示並附 TPI。舊制標準，仍見於舊型設備。

螺距與 TPI 轉換

例如：M8 × 1.25 mm 螺距 = 25.4 / 1.25 = 20.32 TPI (非標準 TPI，證實無法與任何英制螺紋直接互換)。

應力面積

其中 d 為公稱直徑，P 為螺距。應力面積用於計算螺栓的抗拉與保證負荷。

⚠️ 重要： 「最接近等效」並不代表可互換。公制與英制螺紋即使直徑相近，螺距亦不同。切勿強行將公制螺栓旋入英制螺孔，反之亦然——這將損壞兩組零件。

ℹ️ 提示： 辨識未知螺紋：(1) 使用卡尺測量大徑。 (2) 使用螺紋規確定螺距或 TPI。 (3) 於上表進行交叉比對。