Bolt Elongation

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my68barracuda

my68barracuda

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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.

DCDFDAD1-274D-4BAE-AA86-BB8A962FB647.jpeg


7E26035F-408B-4742-BD91-81AA3E253D65.jpeg
 
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That bolt was very likely over torqued at some point in its life. I'm surprised it didn't snap due to being a high grade fastener. 65'
 
Phreakish said:
I hope this doesn't mean the stretched one is currently in your diff?
Click to expand...
Not being able to pull up to torque a was a red flag for me, don't want a bearing cap walking around in a differential.
I have a spare 741 case so I pulled one out of that. That 741 is a peg legger, don't believe it was gotten into previously.
 
It’s also caused by friction in the threads if they are dirty or the fastener is assembled dry. The true torque reading will come when the threads are clean and the proper thread lubricant is applied. That is assuming the fastener hasn’t previously been over stressed.
 
The load will always be transfered to the first 3 or 4 threads closest to the shank. If the bolt only has 1 or 2 threads showing then the same load is transferred to fewer threads causing a failure. Over loading the bold (too much torque) will also cause stretching/failure. If you look at the pics posted by "my68 barracuda" you'll notice that the threads that were stretched were the ones that are closest to the shank! Threads inside the threaded hole do not (should not) stretch. The more threads that are showing equals more threads with which to distribute the load.......but you still need to keep within torque specs!!!
 
This is why I much prefer beam-type torque wrenches, or ones with the digital readout that beep. You can watch the torque increase as you tighten the fastener so you know exactly where it is (not super precise of course depending on how the scale is printed) but you don't have to depend entirely on the internal spring in a clicker-type to give you the right torque. Although I agree with others that bolt was likely over-torqued at some point in its life.
 
Daves69 said:
Someone put some hurt to it. Look at the direction of the raised burr that happen when a tool slipped or failed.
Click to expand...
The raised burr is from me while using worn 12 point socket I put a 3 inch extension on the snap on clicker torque wrench, thinking the additional height would give me a better angle, obviously it did not. I then changed to the 6 point deep impact socket. The deep socket gave a better angle and a secure grip on the bolt head.
And yes I agree the bolt was over torqued and most likely I am the one that did it, as I have had this diff apart three times over the past 10 years.
Tying to figure out how I did it.
I too like beam style wrenches. You know they are right. I don’t have a digital torque wrench, I will look into one of those.
Thanks.
 
That's seen an impact wrench at some point. A LOT of impact wrench.
 
Did not think of that, probably is my wife, sneaking out to the garage and working on stuff when I’m not around.
You met her, lol x 2
 
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