How to Inspect and Maintain Your Trailer Kingpin (Before It Needs Replacing)
A practical guide to keeping your truck and trailer safely connected
Published on
June 4, 2025 at 10:18:11 AM PDT June 4, 2025 at 10:18:11 AM PDTth, June 4, 2025 at 10:18:11 AM PDT
For fleets, service managers, and even owner-operators running their own maintenance schedules, kingpin upkeep isn’t just about compliance. It’s about protecting the integrity of the entire coupling system, reducing wear on costly fifth wheel parts, and ensuring safe, predictable handling under load.
This guide walks through the real-world causes of kingpin wear, signs of damage to look for, inspection steps that can help catch problems early, and how to maintain your kingpin so you don’t end up sidelined by something preventable.
Why Kingpin Maintenance & Inspection Matters
The kingpin is the primary connection between a tractor and trailer, and it's recommended to perform kingpin maintenance and inspection every three months or 30,000 miles. In North America, most kingpins are designed to meet SAE standards** and any trailer with a kingpin showing 1/8" of diameter wear should be removed from service until repaired.
A worn kingpin can create coupling issues like excessive play in the lockjaws or throw off trailer alignment. Kingpin issues can also lead to premature wear and damage to your truck's fifth wheel components – and vice versa. If not removed from service, just one bad kingpin or one bad fifth wheel within your fleet can create a domino effect that can potentially cause premature wear to every truck or trailer you couple across your fleet.
How Kingpins Wear Down Over Time
Even with good maintenance, kingpins are wear components, and they see a lot of action over their service life. If a trailer is coupled and uncoupled frequently or often handles loads in rough conditions, that wear can show up faster. But not all wear is the same, and understanding what causes it helps you spot problems earlier and build more effective inspection routines.
Primary Causes of Kingpin Wear
Standard and routine operation of the trailer is often the main contributor to kingpin wear, slowly building up over the kingpin’s natural lifespan.
Natural Wear from Service Life
Every time a trailer connects to a tractor, the kingpin slides into the fifth wheel lock jaws. Plus, the kingpin is constantly under stress within the fifth wheel hitch for every turn, push, and pull, not to mention every bump in the road. That repeated metal-on-metal contact — even under well-lubricated conditions — gradually wears down the size and diameter of the kingpin shaft. It’s a slow process, but on high-mileage trailers, that gradual wear adds up.
Frequent Coupling and Uncoupling
Frequent coupling and uncoupling speeds up natural wear on the kingpin shaft, especially if the fifth wheel lock jaws aren’t engaging smoothly or parts are poorly lubricated. Drop-and-hook truck operations often involve multiple couplings per day, and every hookup means more load cycles and more impact between the kingpin and fifth wheel assembly.
To be compliant with SAE standards, kingpins should be replaced if the shaft has 0.125" or 1/8" of diameter wear along any point. According to some repair shops, new kingpins often average .005" to .010" of wear within the first year of service. At that rate, you might expect the kingpin lifespan to be anywhere from 12 to 25 years, but technicians say trailers can require kingpin repair within just five years of service due to increased stress factors.
Contributing Factors That Accelerate Kingpin Wear
In addition to natural wear, environmental and maintenance issues can also increase the rate or severity of kingpin wear and damage, often by compromising coupling mechanisms, degrading contact areas, or causing misalignment.
Misaligned or Rough Coupling Techniques
High-hooking, angled/unlevel approaches, or hard contact during coupling puts extra stress on the kingpin. Over time, it can result in uneven surface wear or other structural damage to the kingpin and entire fifth wheel assembly. Hard coupling techniques can also leave the kingpin bent and damage upper coupler plates and weld areas. A kingpin that's bent by just 1° is not operationally safe and must be replaced, as it may prevent proper engagement with the fifth wheel lockjaws or compromise support of the trailer.
Dry or Improper Lubrication
Without proper lubrication, kingpin wear becomes more aggressive since metal-on-metal friction increases between the lockjaws and kingpin. Be sure to check your part manufacturer's guidelines, as grease points can vary depending on the fifth wheel model. However, even "low-lube" style fifth wheels typically require a water-resistant lithium-base grease applied to the kingpin and lockjaw's contact area.
Environmental Exposure & Old Grease
Road grime, de-icing chemicals, and moisture all accelerate rust, corrosion, and metal fatigue. If old grease builds up and traps grit around the fifth wheel or kingpin shaft, abrasion increases. Trailers operating in winter, coastal regions, or rough road conditions are especially vulnerable to this kind of degradation.
Fifth Wheel Slack or Component Wear
A worn fifth wheel with excessive play in the lock jaws amplifies every bump and shift during transit. That movement transfers back into the kingpin, increasing vibration, hammering, and metal fatigue.
Signs of Kingpin Wear or Damage
Kingpin wear doesn’t always announce itself clearly, but the signs are there if you know what to look and listen for. Whether it’s through operational feedback or visual inspection, catching these issues early can prevent more extensive coupling system wear or an unsafe connection.
Want to learn more about the kingpin's relation to fifth wheel play? Check out our blog post on the common causes of fifth wheel play and how to fix it.
Inspecting Your Trailer’s Kingpin
To truly monitor wear and catch early signs of kingpin damage, routine inspections should be part of your regular preventive maintenance program. For each inspection, you'll also need the right tools, right process, and right expectations.
When to Inspect You Kingpin
It's recommended to thoroughly inspect the kingpin seasonally every three months or 30,000 miles. However, as we mentioned earlier, some operational and environmental factors can accelerate kingpin wear and damage. In these cases, the kingpin may require more frequent inspections for safe operations, and its overall lifespan may be reduced.
Generally, you should plan for kingpin inspections as follows:
During every scheduled PM service.
After driver-reported coupling irregularities.
When other fifth wheel and upper coupler parts are damaged.
When trailers experience frequent drop-and-hook operations.
Visual and Structural Damage Inspection
Before you begin the inspection, thoroughly clean the kingpin. Old grease, dirt, and grime buildup can hide critical signs of damage. Then, visually inspect the kingpin and upper coupler plate for wear and structural issues, such as:
Nicks, burrs, gouging, flat spots, or other deformation.
Pitting, rust, or other corrosion from environmental exposure.
Cracks, including near the weld joint or bolster plate.
Separation or misalignment between the kingpin and trailer frame.
Obvious off-center placement or plate warping.
If you see any distortion or damage — especially around the welds and bolster plate — the trailer should be flagged for further inspection or pulled from service.
Kingpin Burrs, Nicks, and Gouges
SAE tolerances for burrs, nicks, and gouges found on the kingpin vary based on the location of the damage (ref. SAE J228). If the damage exceeds the SAE standard, the kingpin must be replaced.
Entire Kingpin: Burrs, nicks, and gouges cannot exceed 0.12" (1/8" or 3.2mm) in depth or 0.25" (1/4" or 6.4mm) in length anywhere on the kingpin.
Head (enlarged, flattened area below the neck of the kingpin): Burrs, nicks, and gouges are allowed, provided they don't violate the rule above.
Neck & Shoulder ("shaft" sections): Max number of burrs, nicks, and gouges of 0.06" (1/16" or 1.5mm) in length cannot exceed 10. Burrs cannot protrude above the metal surface.
Kingpin Wear Limits
Per regulations set in SAE J228, if a kingpin has more than 0.125" or 1/8" of dimensional wear along the diameter of the shaft neck or shoulder, it must be repaired or replaced. When checking the diameter, be sure to measure the kingpin at multiple points and directions along the entire shaft. The chart below shows SAE specifications for a new kingpin.
| SAE Kingpin Specifications | ||
| Kingpin Type | Neck Diameter | Shoulder Diameter |
| 2" (SAE J700) | 2" | 2.88" |
| 3.5" (SAE J848) | 3.5" | 4.48" |
*For reference: The kingpin neck is thinner, tapered section of the shaft that makes contact with the fifth wheel lock jaws. The shoulder is the larger, upper area of the shaft that will sit within the throat of the fifth wheel.
Measuring Kingpin Diameter Wear
To measure for dimensional loss or wear, you'll want to use a no-go gauge or calipers. The SAF-HOLLAND Kingpin Wear Gauge is a universal no-go gauge designed specifically for 2" and 3.5" SAE kingpins. It lets you easily inspect the diameter of the shaft neck and shoulder (as well as squareness and height) to determine if the kingpin is still within safe-operating specifications. For exact measurements, you'll need to use calipers.
Using the kingpin wear gauge, place the center opening of the gauge over the kingpin shaft. Next, using the applicable 2" or 3.5" side of the gauge to match your kingpin type, try to slide the kingpin neck into the designated neck slot of the gauge.
If the kingpin is still good, it should not slide into the slot.
If the kingpin does slide into the slot on the gauge, it is below the 1/8" wear limit and should be replaced.
Maneuver the gauge around multiple points of the kingpin neck to inspect all areas. Then, repeat this step for the shoulder section of the kingpin using the designated shoulder slot on the gauge.
Kingpin Squareness and Height
Kingpins should be flush and perpendicular to the upper coupler plate, mounted at a 90-degree angle for proper alignment between the the tractor and trailer. If the kingpin is off-angle or bent by more than 1 degree, you'll need to replace the kingpin and potentially the upper coupler plate as well.
You can also use kingpin no-go gauges to quickly inspect the angle and height of the kingpin. On the SAF-HOLLAND Kingpin Wear Gauge, these slots are located on the sides of the gauge for 2" and 3.5" kingpins.
Upper Coupler Plate & Mount Inspection
The kingpin is only as stable as the bolster or upper coupler plate it's mounted to. As an initial step, try to move the kingpin by hand. If it moves or wiggles in any direction, that's a sign of structural damage and will require replacement. Every inspection should also include a close look at:
Weld seams for cracks.
Missing or loose screws.
Flatness of the plate surface.
Corrosion or cracking along the frame and mounting areas.
Measuring Plate Flatness and Warping
New upper coupler plates are flat, with the kingpin mounted in the center at a 90° angle. However, the stress of the the kingpin being pulled during its service life can lead warping of the plate. This can potentially affect the positioning/height of the kingpin or cause damage to the fifth wheel by preventing the trailer load from resting evenly on the top plate.
Use a 48" straight edge or a right-angle square to check for warping along all directions of the plate. You'll then need to measure the amount of gap/deflection of any bumps and valleys you find. If the deflection amount exceeds the wear limits listed below, the upper coupler plate will need to be replaced.
0.06" of concave/upward deflection near the kingpin when measured across a 38" diameter.
0.12" of convex/downward deflection within a 10" radius from the center of the kingpin.
0.25" of convex/downward deflection within a 19" radius from the center of the kingpin.
Need help visualizing these inspection procedures and measurements? SAF-HOLLAND has a handy video explainer that makes it easy.
Other Fifth Wheel Parts and Components
Kingpin wear can be a warning sign of other fifth wheel issues too, especially the lock jaws. Any damage or wear within the fifth wheel assembly can mask or amplify kingpin problems as well. While inspecting the kingpin, be sure to examine the fifth wheel lock jaws and locking mechanism for:
Uneven wear or signs of misalignment.
Difficulty locking, unlocking, or holding engagement.
Excessive play after coupling.
If you’re seeing wear on both components, inspect the system as a unit, not in isolation.
When Maintenance Isn’t Enough: Know When to Replace
Not every kingpin can be preserved with upkeep. If a pin or coupler plate is past spec or showing structural damage, the only safe option is removal and replacement. Continuing to run a compromised kingpin doesn’t just risk downtime. It can potentially lead to coupling failures, trailer separation, damage of the fifth wheel assembly, or citations during DOT inspections.
Replace the kingpin if you observe:
Out-of-spec dimensions below SAE and manufacturer wear limits.
Visible cracking, corrosion, or deformation, especially around the weld area or bolster plate.
Any structural damage that undermines flush seating or alignment, such as a bent kingpin by 1 degree or more.
Shop Trusted Kingpin Replacement Parts
Whether you're managing a fleet or maintaining your own equipment, keeping tabs on kingpin condition helps ensure smooth coupling, safer hauls, and fewer headaches at the shop or roadside.
When it’s time to replace, don’t settle for unknown quality. At JIT Truck Parts, we stock OEM and aftermarket replacement parts for the entire fifth wheel assembly, including brands like SAF-Holland, Jost, and Fontaine. Find the perfect-fit trailer kingpin, bolster plate, or other fifth wheel part you need built to meet or exceed OEM standards.
Additional Resources:
- SAE J700 – 2" Kingpin Specifications
- SAE J848 – 3.5" Kingpin Specifications
- SAE J2228 – Kingpin Wear Limits