lock 100 also includes a screw 108 and a plug 110 the bearing member 102 which is shown in fig1 has a generally cylindrical body portion 112 sized to locate within the tooth recess 78 of the tooth 70 the body portion 112 has a first side 114 oriented in use towards the outside of the tooth 70 and a second side 116 oriented in use towards the cavity 72 the first side 114 includes a centrally positioned keyed recess 118 extending into the body portion 112 an engaging portion 120 is located on the second side 116 extending outwardly from the body portion 112 the engaging portion 120 has a generally annular outer face 122 which is angled relative to the sides 114 116 of the body portion 112 the engaging portion 120 thus has an outside wall 124 and an inside wall 125 which extend at an angle of about 75 u00b0 to 80 u00b0 from the second side 116 of the body portion 112 the outside wall 124 and inside wall 125 both extending between the second side 116 of the body portion 112 and the outer face 122 the height of the outside wall 124 and inside wall 125 vary circumferentially about the outer face 122 although the outer face 122 has been described as generally annular the annulus is not circular it includes an introductory portion 126 the introductory portion having an outer edge that is part of the outside wall 124 including both a part cylindrical portion having a radius close to the radius of the body portion 112 and a substantially straight portion the outer face 122 also includes a spiralling portion 127 which gradually increases in radius through about 300 u00b0 from.