Carrillo Conrod Bolt Stretch Gauge

Stretch gauge for measuring the length of conrod bolts during assembly and re-assembly.

The recommended torque spec for a bolt is designed to stretch the bolt to just short of its yield point. However most of the applied torque on a bolt is actually used just to overcome friction. The remaining torque is applied to accurately stretch the bolt to just below its yield point.  Accurately reaching a specified torque figure is affected by friction between the threads, the condition of the threads, and the smoothness of the interface between the underside of the bolt head and the connecting rod, the material strength of the bolt, the lubricant used, and the accuracy of the torque wrench being used.

Measuring the bolt length before tightening and after torque is applied allows you to accurately reach this figure. Some very high tensile bolts like Carrillo CARR bolts can be re-used indefinitely provided the stretch is measured and checked against the bolt specification each time.