22 just described and will control the longitudinal motion of the actuating shaft 48 fig4 referring now to fig1 22 the construction of the knuckle link assemblies will be explained the sensor pad assembly includes a generally elongated cylindrical housing 270 carrying a radioactive source assembly 275 and a detector package 276 the detector package is connected via appropriate wiring to a connector 278 disposed in the uphole end of the sensor pad housing the source 275 and the detector package 276 communicate through the contact 34 windows 34 202 204 and 206 to the exterior of the sensor pad where there are two detectors the downhole end of the sensor pad assembly 30 has a downstream link joint 176 attached thereto by the mounting pin 178 the aperture 274 in the other end of the link 176 accommodate the mounting pin 180 transversely disposed in housing 46 39 fig1 the link 176 has disposed therein a leaf spring assembly 280 one end of which is mounted within the link 176 by a retainer block 282 fastened in place by screws 284 the free end 285 of the spring 280 held by a retainer member 286 attached to the sensor pad housing by a screw 287 the leaf spring assembly 280 normally tends to bras the link and sensor pad 30 outwardly or to an extended position as shown in fig1 the loading force that the leaf spring 280 exerts outwardly on the sensor pad 30 can be varied by adjusting the screw 287 which moves the retainer member with respect to the sensor pad housing 270 and the spring free end 285 the upper knuckle joint link assembly 162 is shown in fig1 22 the knuckle joint assembly 162 comprises an upstream connector block 299.