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Questions regarding reuse of leaf spring suspension U-bolts

Questions regarding reuse of leaf spring suspension U-bolts

Questions regarding reuse of leaf spring suspension U-bolts

Hi everyone,

I would like to ask some questions and start a discussion related to U-bolts used to clamp leaf spring stacks on the suspensions of vehicles such as small to full size pickups, jeeps, and larger work trucks.

Generally, most after-market U-bolt manufacturers recommend to never reuse U-bolts. I think it is  good practice to use new fasteners whenever possible, but I wonder what the position of the auto manufacturers is on the issue and whether it's necessary to replace a U-bolt simply because it has been properly torqued to specs once. At the following link, you can download a small PDF product information sheet authored by Dayton-Parts who produces aftermarket U-bolts...


In the product info sheet it states to not re-use U-bolts. It also states "A Previously torqued U-bolt will suffer from distorted threads from engagement of the deep nut. Deep nuts should be tightened and re-torqued, never loosened and re-tightened." Dayton-parts also recommends in the product info sheet to lubricate the washers and U-bolts with oil or anti-seize compound to reduce friction when torquing the U-bolts nuts.  

LH rods who supplies threaded U-bolt rod to Dayton-parts and others, comments on the issue of U-bolt reuse at the following link...


At the link above, part of what LH rod says is that "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 its 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 retorqued."
The specs for the threaded U-bolt rod used by LH rods are shown at the link directly below...


Basically, the U-bolt rod material is made from a cold drawn, stress relieved, micro-alloyed, 1541 modified steel that  meets SAE-J429. The proof load is 120,000 PSI, the minimum yield  strength is 130,000 PSI, and the minimum tensile strength is 150,000 PSI.

Is there any significance to the issue of the U-bolt threads being rolled and the high-nut threads being cut? Does ASME or any standards address the issue of compatibility between rolled threads and cut threads ? I thought they were compatible and aren't most threads rolled anyway? The issue of a fastener not producing the same clamping force upon a second use would seem to apply to any threaded fastener regardless of whether the treads of the bolt are rolled and the threads of the nut are cut.  Fastenal comments on the issue of fastener reuse at the following link under "reuse of fasteners"...


Fastenal states that "On a demonstration with a 1/2-13 zinc plated SAE J429 Grade 5 hex cap screw and zinc plated SAE J995 Grade 5 hex nut with an installation torque of 70 ft-lbs to obtain a clamp load of 9000 lbs (without any added lubrication). On the second installation, this torque had increased to 95 ft-lbs to obtain 9000 lbs. By the fourth installation, we required 145 ft-lbs to reach a clamp load of 9000 lbs."   

I have seen several threads online regarding the re-use of U-bolts, most are at forums devoted to trucks and the installation of lift kits and such. One such thread is at the link directly below.


In my search online I found one thread where a person claimed to have contacted several auto dealers and was told that U-bolts are serviceable re-usable items. This brings the question to mind, is there any significant difference between OEM U-bolts and aftermarket U-bolts meeting the specs mentioned above? I had always thought factory OEM U-bolts were hardened, however, I was recently told that factory U-bolts have not been hardened for about 30 years now. At another thread, one person who claimed to work for a spring shop said that they reuse U-bolts as a common practice.

It also seems there may be some misconceptions regarding U-bolts on the net. I have seen it posted that U-bolts are actually "torque to yield" fasteners. However, a typical mid to full size pickup truck will use around a 9/16" OD U-bolt and a typical nut torque of 81 foot pounds. It seems to me that 81 foot pounds on a 9/16" OD bolt is not going to produce a stress that will be anywhere near a proof stress of 120,000 PSI so are U-bolts really considered "torque to yield" fasteners ?  I'm not crazy about the idea of a torque to yield fastener in any event.

Perhaps some confusion comes from statements like a reused nut will not "hold torque" or "cannot be adequately retorqued". Perhaps it would be better to say that more torque is required upon each subsequent reuse in order to create the same clamp down force as achieved on the first use. Statements such as not being able to "hold torque" also seem to perhaps create a misconception that the nuts are likely to loosen or back off on reuse. How likely is this ? If the threads are indeed slightly distorted, wouldn't this actually help prevent the nut from loosening once it is tightened down ?

It seems the main reason for not re-using a U-bolt, provided it has never been heated, you can still run the nut down on the threads easily, and the parts have not been overstressed, is that the spring clamping force will be reduced  upon reuse of the U-bolts. A typical 9/16" OD bolt torqued to 81 foot pounds will have approximately 8,640 pounds of clamp down force. Each U-bolt will have two bolt shanks pulling down, so you have a force of 17,280 pounds pulling down per U-bolt, on each side of the axle-housing spring perch or platform. A force of 34,560 pounds per axle side or for each dual set of U-bolts seems huge to me unless I calculated something wrong. Of course it will vary by vehicle, but say for a full size pickup, in general,  how much clamp down force is really needed to prevent leaf spring breakage, rear end shift, or spring stack center bolt breakage ?  

I had read once that brand new identical bolts torqued to the same specs can have tensions that vary by as much as 50% due to friction differences http://www.surebolt.com/. When you factor in that torque wrenches themselves are not 100% accurate and consider all of the variables that can effect the clamping pressure of a bolted joint, if clamp down force is indeed critical, it seems you cannot rely on nut torque or a torque wrench anyway. You really need some way to measure axial clamping force, or you have to design the system in such a way that you will get adequate clamping force regardless of  the variables, and regardless of a few reuses. This seems like the only sensible course of action, but is this how U-bolts for leaf springs are designed ?

Then there is the issue of the common recommendation that new U-bolts be re-torqued after a certain amount of driving. Most recommendations are to drive the vehicle for 100 miles and then re-torque the U-bolt nuts. However, it seems factory OEM U-bolts are never re-torqued, certainly not after any significant amount of driving. Eaton springs recommends to "re-torque the newly installed u-bolts after 50 miles of driving. Then recheck after another 50 miles. Then again after 500 miles. Finally, recheck the u-bolt torque every time you are under the vehicle". That seems a bit excessive to me.  


However, isn't re-torquing the nuts at least somewhat similar to reuse? If the threads have been slightly deformed after the first initial torquing of the U-bolt nuts, then when  you go to check the torque, you are not really checking the torque (or more importantly the clamp down pressure) because it will take more torque to create the proper clamp down pressure than it did upon the first tightening of the nut. If you were to loosen the nut a little before each re-torque, what would that really change or hurt as compared to just re-tightening the nuts each time ?

Scenario 1

I read one thread where a person replaced the leaf spring U-bolts and then had to loosen the brand new U-bolts to make an adjustment on the rear end or something. Provided that the new U-bolts were torqued to specs with a hand torque wrench, the bolts were never heated or overstressed, the proper tightening sequence was followed, the vehicle was not moved or driven, and the nuts could be easily run back down on the U-bolts threads until contacting the lower anchor plates the second time, is this type of one time reuse most likely OK ?

Scenario 2

I read at another forum where a person had some new U-bolts installed on his vehicle for one year and wanted to know if he could re-use them after doing some spring work. Aside from the fact that the U-bolts will have been installed on an in use vehicle for 1 year, & as long as the same bolt and nut conditions exists as in scenario 1 above and there is no significant rust present, is this type of one-time  re-use most likely OK ?

I had a spring shop tell me once that the main reason they recommend to not re-use U-bolts is because people heat up the rusted bolts/nuts to take them off and then reuse the bolts after they have been heated. That would not be a good thing to do since it weakens the parts.  

Of course the simple answer is to just replace the U-bolts, but I like to learn the details and underlying issues. Plus, there are at least a few scenarios as mentioned above where it would be desirable to reuse the U-bolts. If there are indeed spring shops out there who are commonly reusing U-bolts as indicated by a forum thread I found on-line, and with people doing rear end, spring work, or lift modifications while reusing U-bolts, do we have unsafe vehicles on the road because of this or is the clampdown pressure of the U-bolts designed to be so great that its not as much of an issue  as all of the warnings instructing to not reuse U-bolts would seem to imply ?

I tend to think at least part of the reason for the recommendations is because most shops would use impact wrenches on the U-bolt nuts which really could over-stress them. Or, they may use heat on the nuts in order to remove them and reuse the bolts.

Any feedback would be appreciated.


RE: Questions regarding reuse of leaf spring suspension U-bolts

Whew!!  You are quite the writer but you covered a lot of soap there.

This is my opinion (and I just finished a big nut and bolt project at my day job so the topic is fresh in my mind) but if I knew the fastener's history and knew that it hadn't been abused - you mention heating, over-torquing with an impact wrench, etc - I would re use if unless there was visible (or feel - touch or nut dragging) evidence of abuse I would not hesitate to reuse it.

The danger in reusing it is if it was overtightened sometime in its life and had been stretched well into the yield region.

I have reused thousands of fasteners, including large u-bolts on tractor and trailer suspensions and I have trashed quite a few too.  I have also wrung of my fair share and stretched several like they were a jominy specimen.

I read the fastenal thing that you related and my only reaction was that it is their business to sell new fasteners not recommend the reuse of old ones.  Part of my experience is on large power plant turbine equipment where bolts are tightened to a point of measuring the stretch with dial indicators.  BUT, before they are reused, they are NDT tested for cracks and inspected for distress.  They would cost in the thousands of dollars each to replace.

But, if in doubt, replace it.


RE: Questions regarding reuse of leaf spring suspension U-bolts

Further to my answer, you can go on Google and find "bolt torque calculators" and do your own calculations re: bolt stress that the torque produces.  If you know the specs of your fastener you can determine what the torque previously used loaded the bolt with respect to % of minimum tensile or % of minimum yield.  If your bolt has been previously torqued in the range of 50-60% of min tensile for low pedigree bolts (grade 5 and equivalent) or up to no more than 70% of min tensile for higher pedigree bolts (grade 8 and up) I'd feel safe in reusing.  However, if your bolts have been torqued so that they were above 80-85% of minimum yield, I'd be heading to the hardware if it was me.

One of the better calculators that you will find out there will give you everything you ever wanted to know about your bolt stresses.  If I was at my day job computer I'd give you the link but since I'd have to go look for it here, I'll just leave that to you.


RE: Questions regarding reuse of leaf spring suspension U-bolts

Hi rmw, thanks for your thoughts.

That was basically my feeling as well, if you know the bolt's history, there are no common-sense visible indications of something being wrong, and the nuts can be run down on the bolt's reasonably easily by hand, then it seems to me you can most likely reuse the bolts & nuts.

The calculators at Fastenal and Engineers-edge were the best looking ones I found. If you happen to think of a better one & you have time, please pass it along.




I would also guess that most fastener joints are designed so that a couple of reuses are insignificant, provided the fasteners look good as described above. This is because anything where clamping force is real critical should probably not depend on a torque wrench to indicate clamping force anyway, unless you have engineered a generous safety factor. There are too many variables that effect bolt tension significantly.

You mentioned working with some large fasteners. I was talking to an engineer once and he mentioned that on some very large bolts and nuts that are so large they probably don't make wrenches for them, the bolts are tensioned by heat. As I understood it, the nut is installed close to it's bottoming-out point, then the bolt is heated, the nut is then spun down all the way, but with no real clamping force, and then when the bolt cools and the length shortens, tension is produced. Not a precise method my any means, but that's basically what the guy told me as I recall.

Regarding Automotive torque specifications, I have never seen ambient temperature specs mentioned in repair manuals , but you would think this may have an effect. Some shops keep things pretty cool in the winter around 50 degree's F. If you torque a bolt to specs in a 50 or 60 degree shop in the winter and then summer rolls around and it's 95 degrees outside, I wonder how much tension the bolt will lose ? The effects could be more pronounced for someone working in their garage or driveway in the winter at 35 or 40 degrees, although that would not be me unless I had no choice but to do it then.

It would not seem to take much length change in the bolt to produce significant tension changes, and when I think how steel can expand just sitting in the sun on a hot day, perhaps it could be significant. On the other hand, plenty of tire lug nuts are torqued in cold repair & tire shops in the winter and they do not then significantly loosen in the summer on a hot 95 degree day. Sometimes people will re-torque a nut after it has been in service and it will feel if as it had loosened a little. However, it seems at least some of this could come from changes in ambient air temperature as opposed to the nut actually backing off. I guess it's impossible to tell unless you actually scribe location marks on everything.  

I guess the bottom line is that any good bolted joint design will take *all* of the variables into consideration and then apply a generous safety factor. It also seems to me that a good design will take a few reuses into consideration, since there will always be people who reuse fasteners even if they are advised not too.    

As for the fastener companies, there are less headaches and liabilities to just say never reuse a critical fastener twice regardless of circumstances. I don't blame them either, I just like to know the reasoning behind things and the bottom line.

Thanks again,

RE: Questions regarding reuse of leaf spring suspension U-bolts

The thing as I see it, based on so many years of building and restoring race cars and vintage machinery...

In the past, the "science" of 'stress analysis' was not nearly as developed as it, perhaps, is today.  Many of the threaded fasteners were designed with a 'worst case scenario' in mind and well 'over engineered'.  I have commonly reused fasteners with no problems.  The caveat is, of course, "if it looks abused, stretched, damaged or is any way suspect" simply replace it. Common sense, yes?

Now for some of the more modern cars, domestic and foreign, the designed in strength limits have been, in my opinion, lowered below acceptable for me to reuse them. E.g., a recent rework of a Toyota Corolla's suspension...wheel bearings looked like something from a child's toy.  Suspension "U" bolts looked more like exhaust system clamps than the rear axle they were used on...

I'm sure I've made my point.  I could list thousands of similar situations.


RE: Questions regarding reuse of leaf spring suspension U-bolts

There's a reasonable chance that OEM fasteners will be some sort of thread-forming self locking parts, in which case re-use is a bit dodgy. They are often used on chassis parts, so the first thing we do is drill them all out and helicoil them.


Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Questions regarding reuse of leaf spring suspension U-bolts


The bolt heating process is just as the engineer described it, except that it is quite reliable if you know how to control the temperature and count nut turns.  By knowing the thread pitch and how much the nut advances each rotation (or fraction thereof) you can know how much you have stretched the heated bolt.

This method was just coming into vogue when I was doing that kind of work and wasn't so common then.  We did have wrenches to fit the nuts and we used sledge hammers to tighten the wrenches.  They were called slugging wrenches or hammer wrenches or if you are a Brit, a flogging spanner.

With the slugging wrench, you snugged the nut up fairly tight and then figured the stretch you wanted and calculated how many flats of the nut that had to pass a point that you marked on the joint and then you flailed away on it until you had stretched the nut the desired travel.  Usually one man pounded with the hammer while another held the wrench in place with his foot.  Not pretty when the hammer man missed his stroke.  An occasional broken leg was a natural hazard.

The heating method is much better, more safe, and more consistent.

Regarding the temperature variations that you mention, unless your fastener and bolted joint are of vastly different material, you normally don't have to worry much about that.  The coefficient of expansion of steel parts being held by a steel bolt for example will expand and contract together with temperature changes so that the joint maintains constant clamping force.  

What you do have to worry about is if one part of the joint heats faster than another in which case you can loose clamping force or overstress the fastener depending on which heats up first and fastest.

Wheels are often re-torqued because the stress will cause "squirming" of the joint which can cause things to loosen up.


RE: Questions regarding reuse of leaf spring suspension U-bolts

Evelrod wrote:
"wheel bearings looked like something from a child's toy.  Suspension "U" bolts looked more like exhaust system clamps than the rear axle they were used on."

I know what you mean, some brake pads on smaller vehicles look like toys, they don't *look* like they could reliably  stop the vehicle, but they do stop. The front end parts and control arms of some of the new full size pickups I have seen are looking much smaller than they used to 15 to 20 years ago. For awhile there, it seemed like body panels were getting thinner every year.  

On an automobile, money, fuel economy, & safety are the bottom line. I long for the days when products lasted a lifetime if you took care of them, although I don't think that's ever been the case with vehicles unless you live out in the far southwest USA where they don't rust. I take good care of things, but when I think about all the products I have purchased over the years that have failed prematurely I get a little cynical about quality & craftsmanship. Some things probably should be over-engineered and fasteners are probably one of them.   

I think the power-trains of vehicles today are reasonably reliable & will last for many miles if you perform all the scheduled maintenance, change all lubricants and filters at specified intervals, etc.. Things should always improve and evolve, but I don't really have a problem with power-train quality or durability in general, although I have seen some real lemons but you're always going to get a few.    

The main problem I have with vehicles is that I feel they should do a better job of resisting rust. Correct me if I'm wrong, but didn't vehicles used to be much worse with respect to rusting-out until the government stepped in and created some applicable laws ? I thought I read this was sometime in the seventies.       

I've seen brake lines rust out on some vehicles more often than they should have. I think they should try to make the fuel system, gas tank and all lines, and the brake lines, so that they will never rust out in a lifetime, just as a matter of safety. Using stainless-steel for the lines may be cost prohibitive but it seems to me they could coat them with something (perhaps epoxy) in addition to the zinc coating they put on them. I don't know if aluminum, copper, or some other material that inherently resists rust would work OK or not as an alternative to steel lines.

On the body panels the zinc coatings alone do not seem to do the job long term. Perhaps If they coat every panel inside and out with both zinc and a good epoxy, this might help. The same thing could apply to the frames for vehicles with frames. It seems there must be some material / coating solution that would still be cost effective and basically render rust an insignificant issue for people who want to keep vehicles long term. They know what areas are the problem areas, so they could focus there first.

I guess the reason they don't make vehicles out of aluminum is just because it's cheaper to make them from steel. I know some buses are made from aluminum as opposed to galvanized steel, but I'm not sure why it's more desirable on buses than automobiles, unless it's mainly for weight reduction. I've seen some front differential housings on full size pickups made from aluminum since the nineties, I guess it's a cost to weight thing there. I'm all for anything that does not rust (for all practical purposes). Personally I would like to see them make the frames, bodies, and everything else they can out of aluminum or materials that for all practical purposes do not rust.   

This looks interesting...


Maybe someday I will see a vehicle that will last a lifetime if you take care of it, & that will require minimal time to maintain.


RE: Questions regarding reuse of leaf spring suspension U-bolts

RMW,thanks for the additional info on the heat / tension method and effects of ambient temperature on fasteners.


RE: Questions regarding reuse of leaf spring suspension U-bolts

Thanks for the bolt-science link Desertfox, that was an interesting little article. A guy from the GM power-train fastener lab is one of the best sources of info on the subject I can think of.

RE: Questions regarding reuse of leaf spring suspension U-bolts


Here is the link I use at work that I mentioned earlier.


I was also going to list the Bolt Science link before desertfox beat me to it.  Look at their FAQ section.


RE: Questions regarding reuse of leaf spring suspension U-bolts

Thanks for the bolt stress calculator link rmw.

There is just one thing on the calculator I'm not quite sure of. I have data for nut factor (k), but regarding the "bolt torque factor" (q), is that generally left at the default 0.5 setting for most calculations ?


RE: Questions regarding reuse of leaf spring suspension U-bolts

Here in PA, anything bolted underneath a car usually has to be torched off and then replaced.

The only fasteners I know need replaced are head bolts becuase they yield. Aside from that the cost to risk ratio usually tells me "Buy new Bolts". Most suspension kits come with new fasteners anyway.

Just my .02$

RE: Questions regarding reuse of leaf spring suspension U-bolts

First off I have never run into a U bolt that was TTY actually the biggest problem is the rust.
The first concern, surface imperfections that cause stress. Then the biggest problem of all, where are the new U bolts comming from? If they are imports, are they the proper spec material?  I think your way better off with the OEM part than to replace it with some mild steel part from over seas. I have seen some case studys of parts imported and the steel chemistry was way off.  

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