The Biggest U-Bolt Question We Get...
Knowledge Center

The Biggest U-Bolt Question We Get...

This question is asked in our shop and on our website on nearly a daily basis. Take 5 minutes to learn why it's not a good idea to reuse your U-bolts.

Why can’t I reuse my U-bolts?

This is one of the single most commonly asked questions out there when discussing leaf springs and related components.  We get a variety of reactions when we advise our customers that reusing U-bolts is never a good idea.  There are some very specific reasons why we stress this point so much, trying to sell you something you don’t need is not one of them.

U-bolts are a torque to yield bolt.  This means when you tighten down a U-bolt to its proper torque spec, the bolt stretches to achieve that level of tightness.  While the U-bolt has some flexibility in its ability to stretch after being loosened, the bolt will never tighten enough to allow it to regain its full torque spec potential.

The next issue with trying to reuse U-bolts has to do with the threads themselves.  When being tightened, the threads stretch out under torque. When this happens, the threads are actually cut and grooved by the nut as the nut is tightened around them.  This means that if the nut is loosened back up or if it is retightened again, it is going to damage the threads on both the bolt and the nut.  This creates an opportunity for the U-bolt to loosen up over time. 

A high-quality U-bolt will be made of high-grade hardened steel and have rolled threads instead cut threads. Rolled threads offer a larger diameter than the shaft of the bolt, resulting in more core material and strength.  Lower quality U-bolts are often made with cut threads which are just cut out of the shaft of bolt resulting in less material thickness.  Cut thread U-bolts are never a good idea to use because the threads tend to strip.  Additionally, this means the U-bolts won’t be able to achieve near the torque spec that a rolled thread U-bolt can. 

The smallest amount of looseness or vibration in your U-bolts will allow for movement between your axle and leaf springs.  If you have suspension blocks, this movement multiplies.  Over time, a small amount of movement will almost always become a larger problem.  It doesn’t take much movement before center bolt heads being to shear or you run the risk of warping your axle seats.  We’ve seen this and much worse happen over the years as a result of reused U-bolts coming loose. 

Another consideration to bear in mind when talking about U-bolts is ensuring you use both high grade, high quality U-bolts as well as using the correct size U-bolts for your application.  We often encounter loose axles and damaged suspensions due to the U-bolts simply being the wrong size.

Remember, your U-bolts are what hold your axle onto your vehicle. Having your axle come loose can not only be very damaging and expensive to repair, but it can also be very dangerous for you and those who share the road with you.  We’ve seen everything from axles that have tracked to the side and allowed the rear of a vehicle to steer itself, all the way to axles that have torn themselves right out from underneath the vehicle they were supposed to be secured to.  You know what these situations almost always had in common?  The U-bolts were either loose, reused, incorrectly sized or low quality U-bolts.