larger diameter portion of the bore 30 is closed by a member 36 interfacing with the headstock is an assembly called the input plunger 60 which comprises four parts u2014 a main body 61 a control pin 62 a plurality of ball bearings 63 and a bias spring 64 the main body 61 is shown in detail in fig2 where it will be seen to be a generally tubular member closed at one end and having an axial bore 68 which receives the control pin 62 the cylindrical wall of the main body 61 is provided with one or more radial bores 79 each of which is arranged to receive a respective ball bearing in such a manner as to permit the ball to run freely within the respective bore consequently the diameter of the radial bores is approximately equal to the diameter of the ball bearing which it receives the control pin 62 which is received within the bore 68 is a cylindrical component having three axially disposed portions along its length the first portion constitutes a tail portion 65 which is of a diameter so as to be a running fit within the bore 33 it is also of an axial length so as to create a space between its external surface and the smaller diameter portion 50 of the bore 30 which space is equal to twice the diameter of the ball bearings the control pin has a further portion which constitutes a head portion 67 of the control pin this portion is a running fit within the bore 68 of the main body 61 and is arranged to form a spring seat for one end of the bias spring 64 the other end of which abuts the closed.