The lower part of the car body is a constant risk zone, exposed to the aggressive effects of reagents, moisture and mechanical shock. When the owner notices through corrosion or deformation after an accident, he is faced with the question of how to change the thresholds on a car in a modern service center or garage. This process requires not only technical equipment, but also a deep understanding of the body structure, since the threshold is an important force element that provides torsional rigidity.
The quality of work performed directly affects the operational safety and residual value of the vehicle. Poor quality welding or poor waterproofing can lead to rapid repeated rotting of the metal or, worse, to destruction of the body geometry during further use. In this article we will analyze all stages of replacement, from dismantling old elements to final painting and protection.
Modern techniques make it possible to restore even heavily damaged cars, returning them to their factory strength characteristics. However, it is important to understand that there is no universal recipe: the approach to replacing classics differs from working with modern high-strength steels. Measurement accuracy and adherence to welding temperature conditions become critical success factors.
Diagnosis of condition and choice of repair method
Before taking active action, it is necessary to objectively assess the scale of the disaster. Often the external condition of the paintwork hides serious problems within hidden cavities. Craftsmen use special probes and endoscopes to determine the actual thickness of the metal and the extent of corrosion. If rust has affected less than 30-40% of the area of the element, it sometimes makes sense to consider the option repair without removal or partial replacement, although this is a temporary solution.
In cases where the metal has turned to dust or the geometry of the fastening is broken, a complete replacement is required. Here it is important to determine what type of threshold is installed on your car: hinged or integrated into the floor structure. Hinged thresholds they are bolted or spot welded from the outside and are easier to change, while integrated ones require cutting out part of the floor and complex joining.
⚠️ Attention: If corrosion has affected the attachment points of the struts or side members, simply replacing the threshold may not restore the original strength of the body. In such cases, it is necessary to troubleshoot adjacent elements and, possibly, straighten them on the slipway.
The choice between repair and replacement is also dictated by economic feasibility. Sometimes the cost of work and materials to restore a rotten element exceeds the market value of the car itself. The table below shows the main signs indicating the need for urgent intervention.
| Sign of damage | Recommended Action | Complexity of work |
|---|---|---|
| Blistering paint, bubbles | Local repairs, cleaning | Low |
| Through holes up to 2 cm | Supply of patches, welding | Average |
| Rot along the entire length, loss of shape | Complete element replacement | High |
| Deformation after impact | Editing on the slipway + replacement | Very high |
Preparing the car and necessary materials
The success of the operation depends 50% on proper preparation. The car must be driven onto a lift or inspection pit, providing free access to the lower part of the body from all sides. Before starting work, all plastic trim, moldings, door seals and, if necessary, doors are removed so as not to damage them during the welding process. Stripping surfaces from dirt, bitumen and anticorrosive material to clean metal at a distance of 10-15 cm from the cutting zone.
The key is to select a new threshold. The market offers stamped elements, which may differ in metal thickness from the factory originals. Experienced craftsmen often recommend buying thresholds with a reserve or ordering production according to drawings for rare models. It is also necessary to prepare anti-corrosion compounds, primers and consumables for welding.
☑️ Preparation for replacing the threshold
Particular attention should be paid to safety precautions. Working with welding equipment and an angle grinder requires protective glasses, gloves and fire-resistant clothing. In the car interior, it is necessary to remove flammable covers and mats, and cover the interior with a fire-resistant screen, since sparks can burn through the floor covering.
Dismantling old threshold elements
The process of removing the old threshold begins with accurately marking the cutting lines. If the element is attached by spot welding, which is typical for most modern cars, it is necessary to drill out all welded points with a special drill. This is a labor-intensive process that requires care to avoid damaging the inside of the spar or the floor. Bulgarian Used only to separate large sections after removing weld points.
When cutting, it is important to follow the direction of the cut so as not to disturb the strength structure of the body. Often old metal has to be cut out in sections, especially if it is heavily corroded. After dismantling the external part, it is often discovered that the internal part of the threshold or amplifier also requires replacement. In this case, access to them is gained through the resulting opening.
What to do with the internal amplifier?
If the internal threshold amplifier is intact, it is thoroughly cleaned and primed. If it is damaged by corrosion, it is cut out and replaced with a new one, since it bears the main load on the rigidity of the body. Without replacing the amplifier, the new threshold will not last long.
After completely removing the old elements, a view of the spar and floor opens up. This area must be preserved: remove rust residues, treat with a converter and coat with epoxy primer. Sealing joints between the floor and the spar - a mandatory step to prevent moisture from entering the interior.
Technology for installing and welding a new threshold
Installation of a new element begins with fitting. The threshold is put in place, fixed with clamps and the gaps with the door and wings are checked. The geometry must be perfect, otherwise the door will close with difficulty or not airtight. Only after precise adjustment do they begin welding. Use the method semi-automatic welding (MIG/MAG) in a protective gas environment, as it provides better penetration and less metal deformation compared to an electrode.
Cook in short seams (2-3 cm each) in a checkerboard pattern, allowing the metal to cool. This prevents the thin sheet metal from overheating and warping. Particular attention is paid to the joints with the front and rear pillars, as well as the wheel arch. These areas experience maximum loads and must be welded efficiently.
⚠️ Attention: Do not weld with long continuous seams! High temperatures anneal the metal, making it soft and susceptible to corrosion. Use a broken stitch in 2-3cm increments.
Use a copper pad under the welding area if you need to weld metal in a hard-to-reach corner. Copper does not stick to the weld pool and helps to form an even seam on the reverse side.
After completing the welding work, the seams are cleaned with a flap wheel to the level of the base metal. Then repeated anti-corrosion treatment of welded joints is carried out. It's important not to forget about internal cavities new threshold, which must be filled with anti-corrosive agent through technological holes, otherwise the element will rot from the inside in a couple of years.
Anti-corrosion treatment and sealing
Replacing thresholds is the ideal time for comprehensive body protection. After installing and priming the external surfaces, it is the turn of the hidden cavities. A flexible spray nozzle is launched through special holes in the side members and sills, and the anti-corrosion composition (often wax or oil based). It should cover the entire internal perimeter, creating a protective film.
External seams and joints are treated with body sealant. It is applied with an even roller and, if necessary, shaped to match the factory pattern. The sealant prevents water and salt from entering the microcracks between the metal and the paint. For the lower part of the thresholds, which are subject to impacts from stones, it is recommended to apply an additional layer liquid plastic or anti-gravel coating.
The quality of the anticorrosion agent determines how long the life of the new threshold will last. Cheap compounds quickly dry out and crack, losing their properties. Professional craftsmen use two-component compounds that remain elastic throughout the life of the car.
Final painting and assembly
The last stage is the restoration of the paintwork. The threshold is puttied (if leveling is required), primed with filler, sanded and painted. The color is selected according to the car code, but taking into account the fading of the main body, so transition tinting is often required. Paint quality must match the factory one to avoid differences in shade and texture.
After the paint has dried, the car is assembled in the reverse order: moldings, seals, doors and plastic interior trim are installed. The final inspection includes checking the operation of the doors, the absence of squeaks and a visual inspection of the quality of the seams. Properly performed repairs make the junction of new and old metal almost invisible.
A high-quality replacement of a threshold is not just “welding a piece of iron”, but a complex of works to restore the geometry, strength and anti-corrosion protection of the body.
Frequently asked questions (FAQ)
How long does it take to replace one threshold?
In a professional service with a slipway and a painting booth, a complete replacement of one threshold with painting takes from 2 to 4 days. In a garage environment without a spray booth, the process can take a week due to the drying time of materials and the step-by-step execution of work.
Is it possible to change the threshold without welding, for example, with sealant?
No, the threshold is a power element of the body. Fastening it with sealant or glue is unacceptable, as this violates the rigidity of the body and safety in case of an accident. Only bolted connections are allowed for decorative trims, but not for the supporting structure.
Do I need to remove the engine to replace the sill?
In the vast majority of cases, it is not necessary to remove the engine. Access to the places where the thresholds are attached is carried out from below the car or from the interior after removing the seats and trim. The engine is removed only during a total restoration of the body “to the skin.”
What is better: repairing the old threshold or replacing it with a new one?
If more than 40-50% of the threshold area is damaged or its geometry is damaged, it is cheaper and more reliable to replace the entire element. Repairing a large number of patches (“patchwork”) is often more expensive in terms of labor costs and is inferior in durability to a new part.