There exists a spiral clearance space in the API round thread casing joint which is effected by the thread tolerance ranges.
API套管圆螺纹自身存在牙间间隙，并形成螺旋状空间间隙带。
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Tolerance class on thread plug gauges is unspecified.
内螺纹用的螺纹量规等级是没有的。
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