底径 meaning in English

bottom diameter
root diameter

Core diameter of thread
螺纹底径
Some kind cylinder liner request measure nine parameters : inside and outside diameters in different positions , width of the boss of the part , hydraulic packing trough bottom diameter , shoulder place fillet partial diameter , and so on
某型气缸套要求测量九个参数:不同位置内外径,凸台宽度,以及水封槽底径和肩台处的圆角部分的直径等。