Bolt Elongation
The way that bolts, particularly critical fastners like rod, head, flywheel bolts provide the clamp load to keep things together is when tightened to specification, the bolt actually stretches and acts like a rubber band that has tension on it.
A bolt in that condition is said to be in elongation. When the bolt is loosened the bolt returns to it free state length. A bolt that is over tightened and over stretched goes into permanent elongation. In permanent elongation a section of the bolt gets thinner. Due to being thinner or necked down, that section of the bolt has a reduced tensile strength and will not support the torque load of the standard sized bolt diameter.
I ran in to this while assembling a 742 8.75 differential. I could not get one of the cap bolts to pull up to the 90 ft lb specification. I could get it to click at 75 ft lbs, but when the wrench was set to 90 ft lbs, it would just continue with a hard turn. That bolt is shown in the photos.
The cause: this is one of the risks associated to using a clicker style torque wrench. Get a bad angle or some random fault where the wrench does not click, over torque the bolt and this is what you can get.
Another possibility, this differential case and the bolts are 50+ years old. Who knows how many times these bolts have been in and out. If I take it down again I will install all new cap bolts.
The way that bolts, particularly critical fastners like rod, head, flywheel bolts provide the clamp load to keep things together is when tightened to specification, the bolt actually stretches and acts like a rubber band that has tension on it.
A bolt in that condition is said to be in elongation. When the bolt is loosened the bolt returns to it free state length. A bolt that is over tightened and over stretched goes into permanent elongation. In permanent elongation a section of the bolt gets thinner. Due to being thinner or necked down, that section of the bolt has a reduced tensile strength and will not support the torque load of the standard sized bolt diameter.
I ran in to this while assembling a 742 8.75 differential. I could not get one of the cap bolts to pull up to the 90 ft lb specification. I could get it to click at 75 ft lbs, but when the wrench was set to 90 ft lbs, it would just continue with a hard turn. That bolt is shown in the photos.
The cause: this is one of the risks associated to using a clicker style torque wrench. Get a bad angle or some random fault where the wrench does not click, over torque the bolt and this is what you can get.
Another possibility, this differential case and the bolts are 50+ years old. Who knows how many times these bolts have been in and out. If I take it down again I will install all new cap bolts.