"球头半径" can be translated as "ball head radius" or "spherical head radius" in English.
"Ball head radius" is a more straightforward and commonly used term, especially in mechanical engineering or manufacturing contexts where spherical ends or joints are involved.
"Spherical head radius" provides a slightly more descriptive alternative, emphasizing the spherical shape of the head.
Both expressions are correct and can be used depending on the specific context and preference.