Tech Tips - Rod Bolt Technology and Preload
There are three methods that are commonly used to tighten rod bolts: using a torque wrench, measuring the amount of stretch and the torque & angle method. It is important to note that a fastener is like a very stiff spring and it must be stretched a specific amount. The material's ability to "rebound" like a spring is what provides the clamping force to keep the rod bolted together. If you do not stretch the bolt enough, there may not be enough clamp load to keep the rod cap in place which could result in broken bolts or spun bearings. On the other hand, if you stretch the bolt too much, you can exceed the yield strength of the fastener which will weaken it and cause it to fail. Either of these two conditions can result in catastrophic damage to your engine. Always follow the manufacturers instructions to prevent damage to your engine.

Using The Stretch Method To Tighten Rod Bolts - Measuring bolt stretch is the most accurate method for tightening rod bolts and insures the correct pre-load. Simply measure the free length of the bolt before tightening, lube the bolt threads and rod spotface. Install the bolt into the rod and tighten until the bolt is stretched the proper amount. K1 Technologies offers an economical bolt stretch gage for this purpose.

Using The Torque And Angle Method To Tighten Rod Bolts - Do not confuse this with the "Torque to Yield" method. Torque to Yield stretches the bolt to a point where it will no longer return to the original length when loosened and requires the bolt to be replaced after each use. When using the Torque and Angle method, you lube the bolt threads and rod spotface, tighten the bolt to a low torque value as prescribed on the instruction sheet then, using an angle gage, turn the bolt a prescribed number of degrees to properly stretch the bolt. This method uses the highly accurate pitch of the bolt thread to control the amount of stretch.

Using The Torque Method To Tighten Rod Bolts - Torque does not measure clamp load and only measures the amount of friction that must be overcome to turn the bolt. The friction of the mating surfaces of the threads, rod spotface and bolt flange change with each tightening. When you consider the fact that different amounts and different types of lubes also change the friction, using the torque method is like trying to hit a moving target that you cannot see. K1 Technologies does not recommend the use of or provide torque values for tightening bolts.