Engine Performance Expo Tech Talk: Fastener Torque Vs. Stretch

When it comes to fastener torque, most of us know that all fasteners have a torque spec — that is an ideal amount of preload, which is measured through the fastener’s resistance to turning. However, once you dive into what is happening when you reach that torque spec is less understood by the masses. In order to explain exactly what is happening, Total Seal’s Lake Speed, Jr. headed to Jasper Engines to film the 2023 Engine Performance Expo segment on torque versus stretch with AERA’s Chuck Lynch and Jasper’s Noah Fritchley.

First, we need to understand a few fastener terms. Torque is how we extrapolate a fastener’s stretch. We know that if it takes X amount of lb-ft to tighten a fastener, that equates to Y amount of stretch in a given fastener. Stretch is exactly what it sounds like — how much the fastener elongates under preload. X amount of stretch in that fastener equals Y amount of clamp load. Now, the term “clamp load” is really the crux of it all. It is what holds the joint together.

Fastener stress vs. strain curve

This stress vs strain curve shows the difference in preload between a standard reusable bolt (as it hasn’t exceeded its yield strength) and a torque-to-yield bolt. While it offers higher preload, it can’t be reused, since it has exceeded its yield strength.

“All the engine cares about is that the joint is held tight enough,” says Fritchley. “Torque is just our way of expressing the clamp load, which is what the engine cares about.” As Speed points out, the torque number can be manipulated through the use of lubricants and coatings, as most of the fastener’s torque comes from the threads themselves. “Torque, by definition is the measurement to resistance of rotation. There are a ton of variables that come into play there,” says Lynch. “There’a s significant amount that comes from under the head [of the bolt contacting the surface] as well,” Fritchley adds.

“We can fluctuate the actual clamp load by about 15 percent based on what kind of lubricant we use when tightening bolts,” says Fritchley. Lynch adds, “That’s one of the reasons we’re seeing torque-plus-angle specs now, even in non-torque-to-yield bolts.” By setting a base torque and then rotating the fastener a set amount, you are ensuring a more consistent amount of bolt stretch by eliminating some of the variables associated with measuring the fastener’s torque.

Using a Godzilla factory head bolt in a fixture, with a load cell and dial indicator, Lynch and Fritchley perform an exceptional demonstration of the interrelation of fastener torque, fastener stretch, and clamp load. Additionally, they exceed the fastener’s yield point, in order to show what happens when a fastener fails due to overtightening. They say a picture is worth 1000 words. Which means this video is worth way more than that. We can explain it in words all day long, but to really grasp the concept, you need to watch the video of bolts being torqued into this awesome measuring rig.

This nifty gadget used in the video shows the interrelation of the different components of fasteners. With the bolt torqued with a torque wrench, the inverted dial indicator measures fastener stretch, while the load cell physically measures the clamping force being applied by the bolt.

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Greg Acosta

Greg has spent nineteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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