The hardpoint's diameter is defined by the inner diameter, as seen in the image.
挂载点的直径就是指它的内径，如上图。
Inner diameter
英语例句库
This refers to the use of a miniature BTE hearing aid that is coupled to an ear insert or earmold (vented or unvented) via a tubing of approximately 0.8 mm inner diameter.
就是微型耳背式助听器用一根内径大约0.8毫米的细导管耦合到耳塞或者耳模（耳模可以开或者不开通气孔）的验配方式。
原声例句
2017级专外有声
On its way to the lungs, air passes from the larynx to the trachea (windpipe), a vertical tube of about 21 to 27 millimeters in inner diameter and about 10 to 16 centimeters in length.
在通往肺部的途中，空气从喉部传递到气管（气管），一个长约21至27的垂直管道，内径约10至16毫米。
英语百科
List of gear nomenclature
（重定向自Inner diameter）
The addendum is the height by which a tooth of a gear projects beyond (outside for external, or inside for internal) the standard pitch circle or pitch line; also, the radial distance between the pitch diameter and the outside diameter.