Kenworth T680 Fairing Brackets:How They Fail andHow to Catch It

Most lower fairing failures on the Kenworth T680 Next Gen are visible for months before anything falls off. They rarely announce themselves; instead, they show up as a panel that is no longer parallel to the truck, which is something almost nobody looks for.

This is a diagnostic guide to that failure: the symptoms and the order in which they appear, what is actually behind the panel, the three mechanisms that take these brackets out, how to do the repair, what to avoid when you buy the replacement, and what we changed on ours.

The symptom ladder

This failure progresses rather than jumping from healthy to hanging. Every truck we have looked at sits somewhere on the same ladder, and knowing which rung yours is on decides whether this is a bracket job or a bracket and panel job.

Stage 2 is the one worth learning to see, because it is free to catch and it is the last stage where the panel is still undamaged. Stage 5 is the one everyone reports, and by then the expensive part is already involved.

The curb story, and why it is not the common one

Raise this in any shop and somebody says it hit a curb. Sometimes that is exactly right: a low fairing in a tight dock is an easy target, as drivers on the forums will readily confirm.

Impact is the loud version of this failure, and it is easy to diagnose because it leaves evidence: scuffs, a cracked corner, gravel rash or a deformed edge. When a panel comes off from a strike, the panel says so.

We photographed two T680 Next Gens two days apart, one moving on the highway and one parked. Neither panel had a mark on it. The paint was clean, the edges were straight, there was no impact damage anywhere, and yet the panel was no longer attached.

That is the failure worth writing about, because it is the one nobody is looking for. It is slow and silent, working on the part for hundreds of thousands of miles without producing a single event a driver would notice. The bracket does not break in front of you; it gradually stops holding, until one day the panel is on the shoulder of an interstate.

What is actually holding the panel on

Behind the lower fairing, the panel drivers usually just call a fairing or a bat wing, sit the reinforcement brackets. They are the structure between the panel and the cab. On the understep section two Kenworth part numbers do the work, A33-1266-100 and A33-1268, and a complete lower understep takes two of the first and one of the second.

We have an A33-1268 on the bench. The molding on the back reads:

KENWORTH · A33-1268 (A77-1184) REV C · >PP-GF40< · 21714AA · CAV 1

The >PP-GF40< mark is the material: polypropylene with 40 percent glass fiber, injection molded. That is a standard, sensible specification for a part in this position, and it is worth knowing because it is the benchmark every replacement should be measured against.

REV C means the part is on at least its third revision. We do not know why it was revised. Tooling wear, cost reduction, mold flow and function all produce revisions, and PACCAR does not publish the reason. We are noting the fact, not assigning it a motive.

The geometry is where it gets interesting

Look at the A33-1268, the narrow one of the two, and you find a span built as two thin walls with open air between them. That is a reasonable way to mold a part, since it uses less material, shortens cycle time and reduces warp on the tool.

The retention clip is the part worth studying. There is no inset slot for it, and nothing on the bracket recesses the clip or locates it sideways. It slides on, and once installed it still has room to move. That clearance is not a defect in the sense of a part being out of tolerance. It is how the part is drawn. But it means the clip arrives with somewhere to travel, and on a truck, anything with somewhere to travel eventually travels.

So the section has three things working against it. The load path through it is narrow, there is a moving steel clip sitting in the middle of that path, and nothing holds the clip still except the fastener.

Three mechanisms, one chain

There is no single cause here, and anyone who claims otherwise is selling something. What can be laid out is the set of mechanisms that apply to a loaded plastic bracket with a slide-on clip seat and a bolt hole in it, and which one the geometry points at. They are not independent: each one accelerates the next.

1. The clip walks its seat

The clip vibrates, and because nothing insets it, it has room to vibrate into. It sits against a plastic part, on a truck, for hundreds of thousands of miles, and every one of those miles it works against the same few square millimetres of material. The clip slowly saws into the plastic. The seat wallows out, the clip gets sloppier, the movement gets larger, and the sawing gets faster, because every bit of material it removes buys it more room to swing.

And the damage does not stay where it started. Once the clip has cut into the plastic, the cut is a crack waiting for somewhere to go, and it goes toward the bolt. This is not us reasoning from the drawing. We have had failed brackets in hand and the cracking is around the bolt area. That is the least forgiving spot on the part, because it is the point the whole load is reacted through. A cut at the clip seat is a wear mark. The same cut arriving at the bolt is the part coming apart. Figure 2 is what that looks like from outside the truck: the top of the bracket is gone and the bolted portion is still sitting there, because the part came away and left its mounting behind.

Nothing snaps. The panel leaves because the thing holding it stopped being tight. This is the mechanism that produces Stage 2, and it is why a panel goes out of level long before it goes anywhere.

2. The plastic creeps out of its fit

Creep is a plastic slowly changing shape under a load that never changes. It is not breaking or cracking; the material is simply moving. Hold a plastic part under steady pressure and it keeps giving ground for as long as the pressure is on, and it does not spring back when the pressure comes off.

That matters here because the clip is held by a dimension, and on a slide-on fit it is the only thing holding it. The plastic is moulded to a size, the clip is made to a size, and what keeps the joint tight is the pinch between them plus whatever the fastener is clamping. Creep eats exactly that. The bracket has not weakened and it has not failed any strength test. The material under the clip and under the fastener has simply given a little ground and is not the size it was, and a clip that was already free to slide on is now free to slide further.

This is the mechanism that gets missed, because it is invisible to the only number most parts are sold on. Strength answers how much load a part can take. Creep answers what the part does while it is taking it, and nothing on a datasheet asks that question.

A note on the two charts that follow, because they raise an obvious question. Neither one compares the materials in this bracket, and the reason is worth stating plainly. Creep is a standard, published property in the injection molding world. It has an ISO test, ISO 899-1, and molders routinely quote a creep modulus at 1, 100 and 1,000 hours, because the people specifying a molded part expect to see it. Creep in glass reinforced polypropylene specifically, including what temperature and notches do to it, has been studied directly in the literature. What is not published is which PP-GF40 grade PACCAR chose, so there is no curve to look up for this particular bracket.

Outside the molding world the picture is worse, and that applies to us as well. Data sheets for engineering thermoplastics in this class almost never publish creep or fatigue data. We reviewed seven, including our own. Every one gives tensile strength, modulus, impact and heat deflection, yet not one gives a creep curve or a fatigue curve. That is a real gap in how these parts are sold, and any shop telling you its bracket resists creep is telling you something its own data sheet does not say.

So what these two figures show is the mechanism itself, measured on unreinforced thermoplastics, which are the polymers research groups actually publish this work on. Both mechanisms are general. Creep happens to every thermoplastic under sustained load, and stress concentration is a function of shape rather than chemistry. What differs between materials is the rate, and the temperature at which it stops being academic, which is the subject of Figure 7. That one is a direct comparison, and it is built on the property every one of these data sheets does publish.

What creep does to a thermoplastic in three hours Measured on unreinforced PETG, room temperature, compressive. Valvez, Silva & Reis, Aerospace 2022, 9(3), 124

3. The clip seat is a notch, and notches end parts early

A notch is any sharp change in shape: a hole, a groove, the root of a rib. Load does not flow evenly around a corner like that; it concentrates there. Every part on every truck has notches, and that is fine, because the notch barely affects how much load the part can take in one pull.

What it affects is how many times the part can take that load. On a truck, nothing is loaded just once. Every part is loaded and unloaded several times a second for a million miles, which makes fatigue life the number that matters, and a notch cuts it roughly in half.

What a 2 mm notch costs Static strength barely moves. Fatigue life halves. Measured on notched unreinforced thermoplastic specimens. Seifollahi & Kabir, Polymers 2025, 18(1), 1

The three run as one chain, in one direction. The clip starts with clearance because nothing insets it. Creep opens that clearance further. The looser fit lets the clip travel further on every cycle, which cuts the seat faster. The cut seat is a notch the part was never drawn with, which cuts fatigue life again, which lets the seat open faster still, and which gives a crack a running start toward the bolt.

That is why this failure stays quiet for years and then suddenly does not. Nothing about it is dramatic at any single point, but every stage makes the next one arrive sooner, and by the time a driver can hear it, most of the sequence has already happened.

It is also why the panel goes out of level before anything else shows. Level is a dimension, and the first thing this chain takes is a dimension.

The cheap copies are the trap, and here is why

Search these part numbers and you will find copies at twenty five dollars and under with no material named anywhere in the listing. They fit and they clip in tight, but that tells you nothing, and it is the single most important point in this article.

Every mechanism above is time and temperature dependent. None of them can be evaluated on installation day. A bracket made from the wrong polymer installs exactly like a bracket made from the right one, holds perfectly through the first week, and then fails quietly over the course of a summer. The problem is not that it does not work; it is that it works at first, which is exactly what makes it dangerous.

The reason is heat, and this is where the cheap materials give themselves away. Creep is not a fixed property. It accelerates sharply as a material approaches its softening range, and every number in Figure 5 was measured at room temperature. A fairing bracket sits low on a truck, behind a closed panel with little airflow, over pavement that holds its heat long after sundown, through every summer the truck works. The panel keeps the sun off it, but it does not keep the heat out.

Heat deflection temperature is the standard published measure of where a polymer starts to lose its shape under load. It is on every datasheet, it is comparable across materials, and it is the fastest way to see whether a material has any margin in this position at all.

Published heat deflection temperature at 0.45 MPa ISO 75, 0.45 MPa. Higher is more margin before a loaded part starts losing its shape.

Cold is the other half of the year, and it is where commodity plastics give themselves away a second time. Impact resistance is the property that matters when a panel takes a pry bar, a slammed door or a pothole at minus twenty. The fair comparison is not the raw number, because the published tests differ between materials. It is how much of its own room temperature toughness each material keeps once it is cold.

Notched impact strength kept in the cold Published cold value as a share of the same material’s room temperature value. Higher means the material stays tough when it is cold.

So a cheap copy can be entirely honest about fitting, and entirely wrong as a part. It will pass the only test most buyers can run, which is whether it clips in, and fail the only test that matters, which is whether it still fits in eighteen months.

We are not naming sellers. We do not have their parts on a bench, we have not tested them, and we will not publish a claim we cannot support. What we will do is tell you exactly what to look for, because these are visible in the listing before you spend anything:

• No material named at all. This is the one that should end it. A seller who makes the part knows what it is made of and will say so in one word. A listing that never names a polymer is either not sure or would rather you did not know.
• The listing contradicts itself. Specifics say one material, description says another. That is not a typo to shrug off; it means nobody at that shop can tell you what you are buying.
• "Durable plastic", "high strength polymer", "ABS-like". These are not materials; they are the absence of an answer, written to look like one.
• No part number. If a listing will not commit to what it replaces, it has not been measured against anything.
• A price that only works one way. The gap between a twenty five dollar bracket and a thirty dollar bracket is not five dollars of profit. Engineering polymers cost multiples of commodity ones per pound, and that difference has to come out somewhere. The five dollars itself is not the point. What it tells you about what is inside the part is.
• Photographs of a rendering rather than a part. A shop that has one in hand photographs the one in hand.

One more, and it is the reason the failure is worth understanding before you buy. A part that fails this way rarely fails alone; it usually takes the panel with it. That is the difference between a thirty dollar mistake and a mistake that costs you the fairing.

How to check yours in five minutes

You need no tools, and you do not need to get under the truck.

• Sight the bottom edge against the step. Stand back, look along the panel, and compare its lower edge to the line of the step above it. They should be parallel. A mismatch here is Stage 2, the earliest sign visible from outside the truck and the one Figure 1 shows.
• Compare left to right. The fastest tell on your own truck, because both sides should be identical and they almost never fail at the same time. Photograph both sides on your phone and look at them next to each other.
• Sight down the trailing edge. From the front corner, look back along the panel. It should stay tight to the body the whole way. A wedge of daylight that opens toward the rear is Stage 3.
• Push on the panel. Flat hand, firm push on the lower fairing near the step. It should feel like part of the truck. Independent movement or a knock means something behind it is loose.
• Weight the step deliberately, standing on the ground. Load it without climbing onto it. Sponginess is the bracket, not the step.
• Look at the panel itself for damage. This one decides the diagnosis. Scrapes and a cracked corner mean something hit it. A clean panel with a loose mounting is wear, and wear will happen again to the replacement if the brackets are not what gets replaced.

Doing the job

It is a bolt-on repair. The panel has to come down to reach the brackets, and that accounts for most of the labor.

• Do it cool, not frozen. Warm plastic is easier to distort when you are prying, and a genuinely cold part is stiffer and less forgiving of a pry. A shaded shop at a mild temperature is the right place for this job.
• Support the panel before releasing the last fastener. An unsupported fairing panel hangs on its own mounting tabs, and those tabs are the expensive thing to break.
• Take a photograph before you take anything apart. The way the panel sits when it is wrong is the reference for whether it is right afterwards.
• Inspect what comes off. Look at the clip seats specifically, then follow the plastic from the seat toward the bolt. A wallowed seat, a clip with no resistance left in it, a whitened area around the seat, or any crack running from the seat toward the bolt hole are all the story of this article in physical form.
• Inspect the panel mounting tabs while you have access. This is the only time you will see them. A cracked tab found now is a repair; found later, it is a new panel.
• Replace the clips if they are sloppy. A new bracket with a worn clip in it is half a repair.
• Do the set. Two A33-1266-100 and one A33-1268 per side, or the complete set. They have the same mileage on them, and the labour is identical whether you change one or three.
• Snug, not tight. Fasteners into plastic do not get torqued down. Overtightening crushes the boss and builds in exactly the sustained load that Figure 5 is about.
• Check the sight line before you put the tools away. Panel edge parallel to the step, both sides matching. That is the finished condition.

What to avoid

• Replacing the panel and not the brackets. The most expensive version of this mistake. The new panel hangs on the same worn retention and arrives back at Stage 2 within a season.
• Zip ties, screws through the fairing, adhesive. All of them add a new stress concentration to a panel that is already fighting its mounting, and a screw through a fairing is a crack with a schedule.
• Doing one side. Both sides have the same miles. The side that has not failed yet is not healthy; it is simply behind.
• Waiting for it to get worse so you can be sure. The progression from Stage 2 to Stage 5 is the progression from a bracket bill to a panel bill.
• Buying on price alone. As covered above, the cheapest listing on the page is cheapest for a reason that will not be visible for a year.
• Assuming a part that fits is a part that works. Fit is an installation day question. Everything in this article is an eighteen month question.

What we did differently, and what it cost us

Every change on our bracket targets one of the three mechanisms above, and nothing on this list is cosmetic.

• Bridged internal span. Where the OEM part crosses that section as two thin walls with open air between them, ours is closed across. The clip seat stops carrying on its own and the load has somewhere to go. Aimed at mechanism one.
• An inset slot for the clip. This is the change we would point at first. Where the OEM clip slides on and keeps its clearance, ours seats down into a slot that locates it, so the movement that does the sawing has far less room to happen in. Take the travel away and the wear that follows from it does not get started.
• Reinforced clip seat. The seat is the notch and the notch is where this failure starts, so the slot has material behind it rather than a thin wall, and the path from the seat to the bolt area is supported rather than open. Aimed at mechanisms one and three.
• Increased wall thickness. More section in the walls that were thin, which is the difference between a notch in a loaded thin wall and a notch in a supported one. Aimed at mechanism three.
• Internal rib structure. Ribs spread load across the part instead of letting it pile into one corner. On the OEM geometry the rib roots, the clip seat and the bolt area are where stress concentrates, and the bolt area is where we have found these parts cracked. Aimed at mechanism three.
• Extra material in the high stress areas. Rather than making the part uniformly thicker, which would only add weight and shrink, we added section where the load actually goes. Aimed at all three.
• Carbon fiber reinforced plastic, chosen for sustained load. Selected for how it behaves under a constant load over years rather than for the highest single number on a datasheet, because the failure in this position is a dimension problem and not a strength problem. Aimed at mechanism two.

The one thing we did not do is make it more rigid in every direction. The OEM bracket is stiff everywhere, and the panel it holds is being pushed by wind all day. Ours gives side to side so the panel can move without fighting its own mounting, and stays stiff up and down, because people stand on that step. It is not loose; it gives where the wind pushes and holds where a boot lands, and every bit of load the bracket absorbs is load the clip seat does not have to carry.

What a failed bracket actually costs

Almost nothing, right up until the moment it does.

The bracket is inexpensive; what it holds is not. A loose panel works its own mounting points, and when the retention finally lets go it usually takes the fairing's mounting tabs with it, which turns a bracket into a bracket and a panel. The step above it stops being something worth trusting with a full weight. And an aero panel standing off the body is not doing the job it was fitted for. Fleets fit lower fairings for fuel economy, and a fairing that is not sitting flush is not earning that back.

Replace the bracket and you keep the panel. Wait, and the panel gets a vote.

Buying a replacement, including from us

Only OEM is OEM, and that includes us. We make an aftermarket part, and so is every other non-dealer bracket on the internet. Anyone implying otherwise has told you something untrue on the first line of the listing.

There are three routes: the dealer part, which is the known quantity at the known price; an aftermarket part from a shop that will tell you what it is made of and why; and an aftermarket part from a shop that will not, which is covered above.

We would rather you buy a good bracket than our bracket. We believe those are usually the same thing, and where they are not, the section on what to look for still works without us.

Part numbers and current pricing

OEM prices are the sale and list prices published by two online retailers, checked 21 September 2026. They move and vary by dealer, so treat them as a recent observation rather than a quote and check your own dealer.

How we looked at this

Bushido Performance makes an aftermarket version of the part this article is about. That is a conflict of interest, and this article should be read with that in mind. What we can put against it is the evidence itself, and a clear line between what we observed and what we think it means. Where we are reasoning rather than reporting, we say so.

So the evidence can be weighed: this article is built on two T680 Next Gens photographed on 27 and 28 August 2026, one moving and one parked, seventeen seconds of video of the second truck, video of the first from which Figure 1 is a single frame, one new OEM A33-1268 in hand, and failed OEM brackets taken off trucks, which are where we have looked at the cracking directly. Every photograph here is ours. The creep, notch fatigue and heat deflection figures are published literature and datasheet values, cited where they appear.

Sources

• Creep, Figure 5. Valvez, S., Silva, A. P. & Reis, P. N. B. “Compressive Behaviour of 3D-Printed PETG Composites.” Aerospace 2022, 9(3), 124. doi.org/10.3390/aerospace9030124 (open access).
• Notch fatigue, Figure 6. Seifollahi, M. & Kabir, M. Z. “Fatigue Life Assessment of Notched PLA Manufactured Using FDM 3D-Printing Technique.” Polymers 2025, 18(1), 1. doi.org/10.3390/polym18010001 (open access).
• Creep in glass reinforced polypropylene, the OEM material class. Kerim Kyzy, B., Lanzutti, A., Magnan, M., Rondinella, A., Simonato, M., Furlanetto, R. & Fedrizzi, L. “Creep study of glass reinforced polypropylene: Effect of temperature and presence of notches.” Engineering Failure Analysis 2021, 128, 105624. doi.org/10.1016/j.engfailanal.2021.105624. Cited for the existence and direction of the work; it is paywalled and we have not reproduced any figure from it.
• Creep test method. ISO 899-1:2017, tensile creep of plastics.
• Heat deflection temperature, Figure 7. ISO 75 values. The figure for our material is the published value from the technical data sheet of the grade we use. The others are representative published values for their material class, not any single product’s specification.
• Cold impact, Figure 8. Charpy notched impact, ISO 179/1eA, at +23 °C and the stated cold temperature, from manufacturer data sheets: ALBIS ALTECH® PP-H A 2040/159 GF40 (12 and 9 kJ/m², −40 °C); a fiber reinforced grade of the same base polymer as ours (19 and 15, −30 °C); INEOS Styrolution Terluran® GP-35, ABS (19 and 7, −30 °C). PETG is Eastman Eastar™ 6763, notched Izod to ASTM D256 (101 and 37 J/m, −40 °C), which is why the figure compares retention rather than raw values.
• OEM material. Read directly off the molding of an A33-1268 in our possession: >PP-GF40<, REV C. Figure 3.
• Driver accounts of lower fairing damage. “Broke Both Fairing,” TheTruckersReport and related owner-operator threads, read for how drivers describe the part and its failures rather than for engineering data.

Common questions

What trucks do these fit?

Kenworth T680 Next Gen, 2022 and up. The part numbers are the reliable check, A33-1266-100 and A33-1268, because trim and model year descriptions vary more than the brackets do.

How do I know if mine are failing?

Sight the bottom edge of the lower fairing against the line of the step above it. If they are not parallel, or if one side looks different from the other, the retention has already moved. That is Stage 2 on the ladder above and it is visible from twenty feet away once you know to look.

Where do I get the brackets?

Ours are the A33-1266-100BP, the A33-1268BP, and the complete understep set, which is two of the large bracket and one of the narrow one. The dealer part is available through any Kenworth dealer under the same OEM numbers.

Do I have to replace all three at once?

No, but they have lived the same life. If one seat is worn, the others have the same mileage on them, and most people doing the job replace the set while the panel is already down because the labour is the same either way.

How long does it take?

It is a bolt-on. The panel has to come down to reach the brackets, and that is most of the work.

Is it really made of carbon fiber?

It is a carbon fiber reinforced plastic, which means chopped carbon fiber through an engineering polymer. That is not the same thing as a carbon fiber layup, and we would rather you know which one you are buying. The reinforcement is there for stiffness and for how the part behaves under a load it carries for years, not for appearance.

Why not just buy the cheapest one?

Because every failure mechanism described here is time and temperature dependent, and none of them can be judged on installation day. The cheap copy fits, holds, and then loses its fit quietly. When it lets go it usually takes the panel's mounting tabs with it, which is the expensive half of the repair.

Do you have a warranty?

90 days. If it does not fit or it fails, tell us and we will sort it out.