All About U Bolts
Types of U-Bolts
Three basic types of bends are used on U-bolts depending on the suspension design and the shape of the mating parts: Round Bend, Semi- Round Bend and Square Bend. Additionally, each of these types of bends may used forged material. This is used primarily where additional clearance is required between the U-bolts and, for example, the frame.
7 Important Questions About Ubolts...
When springs repair or replacement is necessary some people give ubolts little thought because they are planning to save a dollar and "just use the old ones" which sounds like a plan until...
Rest assured your disappointment will be amplified if you are doing the job on the weekend when everything is closed or if you are a long way from the supplier. So to avoid being disappointed by any of these possibilities, let’s just say it is a wise practice to REPLACE the ubolts when a spring removal or replacement has been done.
It is also recommended that the ubolts be re-tensioned 7-10 days after installation as the spring settles into position and any initial stretching of the ubolt threads has occurred. The re-tension will ensure the leaf spring remains firmly secured and is not allowed to rock in place causing unwanted spring flex in the middle of the spring which can cause leaf breakages to occur. Loose ubolts can also cause a leaf spring to walk out of position which will damage your centre bolt and affect the alignment of the vehicle.
Remember….. centre bolts hold the spring together for assembly and help locate the spring onto the axle in the correct position. Correctly tightened ubolts are what holds the springs firmly in place on the vehicle when driving.
So now that you know WHY you need to replace the ubolts, let’s have a look at what we need to know about the ubolt when we are ordering new ones.
Most times just supplying the vehicle make, model and year will be enough. Other times the vehicle or spring specifications have changed and you need to describe the physical properties of the ubolt.
1. What shape is the ubolt?
The middle of the ubolt will be formed to go over or under the differential or the mount plate and it will most times be one of a few basic shapes square, round and semi-round.
Pay particular attention to the difference between round and semi-round. Seen together it is much easier to tell the difference. See that the round one is almost a perfect semi-circle shape whilst the semi-round one has only a slight arch.
Further variety exists still if the shape has been forged from round material to be flat so we then have square forged, round forged or semi-round forged. There are also some semi-round designs that are vehicle make/model specific and special dies are required to achieve these shapes but they are not so common.
2. What size material has been used?
The material used to make the ubolt will be round because the end of the ubolt needs to have a nut thread on to it. So when you are asked what material, you will need to measure the diameter of the rod AT THE THREAD. This is because most ubolt threads are rolled, not cut and the rolling process will make the thread measure as a bigger diameter than the rod itself and if you need nuts to suit measure the thread size …. not simply the rod size.
3. What is the inside measurement of the ubolt?
This measurement is critical because it must be the correct width to go over the diff housing or spring pack. Too narrow obviously won’t fit on and too wide won’t secure the spring correctly or fit into the holes of the spring mount cradle or plate. Measure from the inside of one ubolt leg across to the other. Always measure at the shaped end of the ubolt as the legs of the ubolt may spread during removal and give you a larger and incorrect reading.\
4. What length are the legs of the ubolt?
When measuring the leg length of your ubolt always measure from the under side of material at the top down to the end of the leg.
Now don’t forget that this is the measurement of the ubolt you have now and you may need to order longer ones if you are adding a packer plate or extra leaf to your spring to add height or strength.
It’s a good tip to have a peak how much thread is sticking out of your original ubolts BEFORE you take them off so you know if your new ones will need to be longer once you have done your additions. Remember that you must have a full nut + two threads extra on the thread of your ubolt exposed after the nut.
Any less and the nuts could work loose. And if you do replace your ubolts,give them a re-tighten approximately 7 days after to make sure any initial settle in of the springs and ubolts has occurred. Ubolts must be tight or problems will occur.
5. How much thread does your ubolt have on each leg?
Most of the time the length of the thread is relevant to the length of the leg with plenty to spare but if you are asked for it, know where to measure.
6. What grade material should the ubolts be made of?
This is not normally something you will need to stipulate. Usually the supplier will know relevant to material size and what the standard requirement is. Some of the smaller 2wd/4wd ubolts are equivalent to Grade 5 whereas the big heavy ubolts are supplied as Grade 8.
7. (6:29) What if the ubolts I need aren’t available off the shelf?
Many suspension suppliers and installers have a ubolt bending machine on site. With the use of one of these machines a ubolt blank can be selected considering the material size, width and leg length you need and bent to the shape or form stipulated for the top of the ubolt.
It is then a case of selecting the right “die” to form the shape needed to make your ubolt.
Individual machines will have limitations on the diameter of rod they can bend. Please remember that threaded rod blanks are a cold bend steel material and it is a definite no-no to use heat on the threaded rod blank to help with the bend. Using heat can cause a reduction in strength or failure of the ubolt.
Hopefully that tells you enough so you can talk ubolt language when asked and you can order your ubolts without too much difficulty.
All these questions answered in this video.
- Used U-bolts will have rusted and damaged threads from the previous installation
- A previously torqued U-bolt will suffer from distored threads from the engagement of the deep nut. Deep nuts should be tighened once and retorqued, never loosened and retightened.
- A used U-bolt may have suffered from fatigue as well excessive stress since achieving accurate torque with commonly used impact wrenches is very difficult.
Removed U-Bolts should NEVER be placed back onto the vehicle, they should be thrown away. Suspension U-Bolts are manufactured with a smooth rolled thread, while the mating Hi-Nuts are manufactured with sharp cut threads. When a U-Bolt is tightened to it's recommended torque level, the U-Bolt threads stretch as they mate with the Hi-Nuts. Although, not always visible to the naked eye, this damages the threads. Removing the Hi-Nuts from the U-Bolt will cause a cross-threading that will not allow the U-Bolt to be adequately re-torqued. A common practice in most maintenance facilities is to use a impact wrench to tighten U-Bolts. Consistent, accurate torque is next to impossible to obtain with a impact wrench, and in most cases an over-torqued fastener is the result. We recommend using a torque wrench when installing U-Bolts. New U-Bolts should be torqued after 2 weeks of usage.
As previously explained the primary function of the U-bolt is to maintain the clamping force on the spring assembly and related parts. Since clamping force is a function of the condition of the U-bolt, the damaged threads of the used U-bolt will make reaching this force very difficult. Considering that reusing U-bolts can lead to premature spring failure and other suspension damage it is clear that U-bolts should never be reused.
Inspect for signs of loose U-bolts
Virtually all leaf spring failures through the center hole are caused inadequate U-bolt clamping. Also look for signs of movement within the area between the U-bolts. Worn or polished surfaces on axle seats or top plates are sure indicators that unwanted movement has occured. Closely inspect and if necessary replace each damaged component.
Retorque the U-bolts
Retighten U-bolts after the first 500 to 1000 miles. As new or repaired springs wear in, some settling of the spring stack will occur. Even a minor amount of this settling can cause a dramatic reduction in U-bolt clamping force. If possible, retorquing of U-bolts should be done with the vehicle under load.
Use recommended torque specifications
Compared to other fasteners, recommended torque levels for U-bolts are significantly lower. This is due mainly to the stress concentration effect of the U-bolt bend itself, the dynamic loading that the spring transmits to the U-bolt and the difficulty of acheiving even stress on both legs of a U-bolt.
Recommended torque levels from the vehicle or suspension manufacture should always be used where possible. If these specifications are unknown, the following chart can be used as a guide
(Dia x Thread)
3/8 x 24
7/16 x 20
1/2 x 20
9/16 x 18
5/8 x 18
3/4 x 16
7/8 x 14
1 x 14
1-1/8 x 14
1-1/4 x 14
Torque U-bolts Evenly
Follow these guidelines when installing U-bolts
Q. Don't you just have a book that you can look up my vehicle's U bolt size in?
A. No, there is no book. There are too many variations when it comes to U bolt sizes. This is because during the lifespan of the vehicle, people can change springs, axles, etc. Also, the manufacturers do not list u bolt sizes.
Q. Is it really important to re-tighten Ubolts?
A. It is very important to re-tighten Ubolts. The first reason, is because they stretch. And if the truck is used a lot everyday with a lot of loads, you should re-tighten the U bolts after 1-2 Days. This can be done on the ground, with a load on the vehicle by hand using a torque wrench (which we recommend). It can also be done in a pit, with an air gun that is heavy enough to do the right torque setting. You can scroll down to see the torque specs below.
Q. Can you overtighten a Ubolt?
A. YES! Be careful that you do not over-tighten your Ubolts. Over-tightening can be just as detrimental as not tightening.
Q. Can I just re-use my old Ubolts?
A. No. We never recommend using your old Ubolts. If you were to re-use an old Ubolt, there could be a problem when you retighten the nuts. The nut tends to grab the old threads, and these threads have been weakened because of exposure to road conditions ie. salt and moisture.
Q. How do I get the exact measurements that you need?
A. You can click "here" to bring you to our measurement page. Just scroll down to the section that reads "Measurements needed".
U Bolt bending brake in action.