Cracked Lift Gate Mounting Plates on Maxon and Tommy Gate
Published March 5, 2026 · 6 min read
Short answer. Lift gate mounting plates crack where the plate meets the frame rail or the rear sill, usually at a weld toe. The gate keeps working while the crack grows. We inspect the plate, gusset and sill attachment, repair the structure with the gate removed, then reinstall so the platform meets the sill flush again.
Why the plate cracks and not the gate
A lift gate is a lever. The platform sticks out behind the truck, the load sits at the far end of that lever, and every bit of the resulting moment has to be resolved back into the chassis through the mounting plates. Those plates are the highest stress components in the whole assembly.
The gate itself is engineered as a system by the manufacturer. Maxon, Tommy Gate and Anthony all build gates that will outlast the mounting if the mounting is right. What varies enormously is how the plates were attached to a specific body by whoever installed the gate, and how many cycles that attachment has seen since.
The failure is fatigue, not overload. Each lift and lower cycle flexes the plate a tiny amount at the weld toe. Repeat that fifteen or twenty times a day for years on a West Covina distribution route and the metal at that transition eventually starts a crack. Nothing dramatic happens the day it starts.
The four places cracks show up first
Cracks are predictable because stress concentrations are predictable. Look where the section changes abruptly.
- At the upper weld toe where the mounting plate meets the vertical face of the frame rail
- At the end of a gusset, where a triangular brace stops and the plate continues
- Around bolt holes in a bolt on plate, spreading from the hole toward the plate edge
- At the rear sill attachment, where the plate ties into the body rather than the chassis
- At the pivot pin bosses on the plate, which see the highest local bearing load
- Along a repair weld from a previous fix, which is the most common location on older units
How to spot a crack before the gate sags
Paint tells you before metal does. A crack under paint lifts a fine line of the coating along its length, and rust bleeds through that line as a thin brown streak. On a plate that has been powder coated, look for a hairline where the coating has fractured rather than chipped.
The second clue is movement. With the platform stowed and someone standing on it, watch the plate at the frame rail from underneath. Any visible flex at a joint that should be rigid means something in that joint is no longer continuous.
The third clue is behaviour. A gate that has started to run out of level, where the platform tilts slightly to one side as it lifts, often has a mounting problem rather than a hydraulic one. The two sides of the mounting are no longer equally stiff, so the geometry shifts under load.
Fleets doing their own walk arounds should wire brush the plate area once a quarter and look with a light. Grease and road film hide hairlines completely, which is why cracks are usually found only after they have grown.
What a proper structural repair involves
Welding over a crack is not a repair. The crack tip keeps running under the weld and the joint fails again, usually faster than the first time because the heat affected zone is now bigger.
The correct sequence starts with finding the actual extent of the crack. We drill a stop hole at the crack tip, which blunts the stress concentration so the crack cannot propagate further while we work. Then the crack is ground out to a full penetration preparation along its length.
The joint gets welded out with the correct process and filler for the base material, and then the geometry that caused the crack gets addressed. That usually means adding or extending a gusset so the section change is gradual instead of abrupt, and moving the new weld termination away from the old one.
Where the plate itself is fatigued across a broad area rather than cracked in one place, the plate gets replaced. Building up a tired plate with weld metal produces a hard, brittle assembly that cracks somewhere new.
Repairing the body the plate is attached to
On many box bodies the mounting plates tie into the rear sill and the understructure as well as the frame rails. If the sill is soft, corroded or has been pushed by a dock strike, no amount of plate repair fixes the problem, because the plate is being asked to resolve its load into something that moves.
We check the sill and the last two crossmembers whenever we have a lift gate off. Corrosion at the sill on a unit that has run coastal or refrigerated work is common, and it is often what actually caused the plate to start flexing.
This is where the two repairs merge. A gate mounting job on an older unit frequently becomes a sill section replacement plus a plate repair, and the estimate has to be written with that possibility stated up front. It is not a supplement anyone likes, but it is better than reinstalling a rebuilt plate on rotten steel.
Removing and reinstalling the gate correctly
The gate has to come off for real structural work. Welding near hydraulic lines, near a pump motor, or near the wiring is not acceptable practice, and you cannot prepare a joint properly around an assembly that is in the way.
Before anything is unbolted, we record the mounting geometry. Plate height relative to the sill, plate spacing, and the platform position when stowed all get measured and photographed. That is what the reinstallation gets checked against.
On reinstallation, the platform should meet the sill flush across its full width when stowed. A gate that closes tight on one side and shows a gap on the other is telling you the mounting is not symmetrical, and that asymmetry is exactly what fatigues plates. We adjust before the truck leaves rather than after a driver complains.
We handle body, structural and equipment mounting work. We do not perform engine or drivetrain repairs on commercial units and we do not issue compliance certifications for any equipment.
Planning the job around a delivery week
A single cracked plate with gate removal, structural repair and reinstallation is typically a two to three day job. Add a day or two if the rear sill needs sectioning, and add refinish time if the unit is painted rather than bare.
Fleets in West Covina reach the Yorba Linda shop in thirty to forty five minutes. The usual run is the 10 or the 60 west to the 57 south, then the 91 east to Imperial Highway, with Carbon Canyon Road and the 71 as the backup when the 57 is stacked.
We do not offer mobile service. Structural welding on a gate mounting has to happen in a controlled bay with the gate off. If you have several units with the same gate model and similar age, it is worth bringing them through in sequence, because fatigue cracks in a fleet show up on the same schedule. Call (949) 799-3387 to talk through a staged plan.
Questions on this
Can a cracked lift gate mounting plate be welded up in our yard
It can be welded, but welding a crack without stopping it, grinding it out and correcting the geometry just moves the failure. It usually comes back within months and the second crack is larger. There is also the practical issue of welding around hydraulic lines and wiring with the gate still mounted, which we do not consider acceptable. The gate needs to come off, the crack needs a stop hole and a proper preparation, and the gusseting needs to be revised.
Is a cracked mounting plate safe to run until the truck is scheduled in
That depends on the crack, and it is not a judgement to make from a photo. A hairline at a weld toe on one of two plates behaves very differently from a crack that has run past a gusset end. What we tell fleets is to take the affected unit off heavy loads immediately, reduce the number of cycles per day, and get it looked at within days rather than weeks. A mounting failure under load is not a repair problem, it is an injury problem.
Do Maxon, Tommy Gate and Anthony gates crack in different places
The gates differ in geometry so the stress concentrations sit in different spots, but the principle is the same across all of them. Column style gates tend to crack at the column base attachment. Rail and tuck away gates tend to crack at the plate to rail weld and at the pivot bosses. Cantilever style gates load the upper plate attachment hardest. We inspect the whole mounting regardless of brand because previous repair welds are the most common crack location on any of them.
Will the gate need to be recalibrated or adjusted after the plates are repaired
It needs mechanical adjustment, yes. Once the plates are back on, the platform has to be checked for flush contact with the sill across the full width, and the stow position has to be reset. If the repair changed the plate height even slightly, the geometry at full lift changes with it. We check platform level at the bed height position and at ground level before the unit goes back, and we correct any tilt at that point.
Does the rear sill usually have to be repaired at the same time
On older units it often does. The plates tie into the sill and the understructure, and if the sill has corroded or been shoved by a dock strike, the plate has been flexing against something that moves. That flexing is frequently the original cause of the crack. We inspect the sill and the rear crossmembers whenever the gate is off and tell you before we start welding. It is a common supplement on units past about eight years of service.
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Ready to get a written estimate
We write estimates at the shop and bill your carrier direct. 30 to 45 minutes from West Covina on the 57 and the 91.