Fulcum Suspension - Why Can't I use my U-bolts?


This is a common question asked when fitting new Leaf Springs. Fulcrum Suspension have put together this handy information below.

When supplying a leaf spring kit, spacer lift kit or a full suspension lift kit, we are often asked whether the original equipment (OE) U-bolts can be reused.

We get a variety of reactions when we advise our customers that new U-bolts should be used, however 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.


Reason 1: U-bolts are often a torque to yield bolt.

This means when you tighten a U-bolt to its correct torque spec, the bolt stretches to achieve that level of tightness. However, the U-bolt can only stretch so far, each time the U-bolt is tightened, it is closer to failure. To us, it’s not worth the risk.

Reason 2: Used U-bolts don’t tighten the same as new U-bolts. 

The next issue with reusing U-bolts has to do with the threads themselves. When being tightened, the threads are deformed under the torque. Each time the U-bolt is tightened the threads are cut and grooved by the nut as the nut is tightened around them. This damage creates an opportunity for the U-bolt to loosen over time. Corrosion, dirt, oil and wear increase friction on the thread, so even when tightened to the correct torque spec, clamping force is decreased and may not securely hold the leaf pack.

Reason 3: The original U-bolts are generally too short for aftermarket leaf springs.

We often encounter loose axles and damaged suspensions due to the U-bolts simply being the wrong size. OE U-bolts are not long enough to fit over aftermarket leaf spring packs and cannot be tightened properly. Aftermarket and/or raised leaf spring packs are typically thicker with more primary & secondary leaves in the pack, this is because most aftermarket leaf springs are designed to carry heavier loads and/or raise the vehicle. Please see the below diagram showing this.

Reason 4: We know our U-bolts are the best. 

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 cut from the shank of bolt resulting in reduced 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 a spacer lift kit, this movement multiplies. Over time, a small amount of movement will always become a larger problem. It doesn’t take much movement before centre bolt heads begin 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.

You know what all the above situations almost always had in common? The U-bolts were either loose, reused, incorrectly sized or low-quality U-bolts.

Thankfully Fulcrum Suspensions will always supply you with the correct U-bolt for your car, whether you are buying a leaf spring kit, a spacer lift kit or a full suspension lift kit. 

Remember, your U-bolts are what holds 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.

When installed the aftermarket U-bolts may be too long as all our aftermarket U-bolts are designed for universal fitment. For a neat and tidy fitting, once installed the U-bolts may need to be cut to suit your leaf spring pack. If it does need to be cut, first ensure you fit the nut and measure how much thread will need to be removed.