The present invention relates to a dental head unit capable of measuring a root canal length of a patient. The head unit includes a head section having a sleeve rotatably supported therein for detachably holding a cutting tool therein, a neck section extending proximally from the head section and having a proximal end for connecting to a handpiece body, a contact capable of detachably contacting the cutting tool for providing electric connection to the cutting tool when in contact, and a contact holder fixed to a proximal end of the contact and axially slidable with respect to the neck section. The contact holder has a lead attachment to which a lead for electrically connecting the contact to a root canal length measuring device is detachably attached. The present invention also relates to a contact holder assembly including the contact and the contact holder.