A bone anchor locking device for receiving a bone anchor having a shank and an enlarged head, the device has a bone anchor, a dome and a conical split washer. The receiving portion has a base extending radially outward and inwardly to a wall. The dome has a center aperture sized to pass a bone anchor. The split washer has an inner conical surface sloped to slide and expand against at least a portion of the base on insertion of a bone anchor to allow a maximum diameter of a head of a bone anchor to pass and thereafter the split washer retracts overlying the maximum diameter of the head to prevent a bone anchor from backing out.