DIY car threshold amplifier: from drawings to installation

Car thresholds are one of the most vulnerable areas of the body, subject to corrosion, mechanical damage and deformation during an accident. Even minor chips of paint on thresholds eventually lead to rust, which corrodes the metal from the inside. Threshold amplifiers solve two key problems: protect against corrosion and increase body rigidity during side impacts. Ready-made solutions from manufacturers (for example, BOSAL or Febi) cost from 3 to 15 thousand rubles per set, but if you have the tools and minimal welding skills, you can make them yourself.

In this article we will analyze the entire process - from the selection of materials to finishing. You will learn which metal is best to use for amplifiers, how to correctly cut blanks according to patterns, which welding technologies are suitable for body work, and how to avoid typical mistake: installing amplifiers without preliminary treatment of the internal cavities of the threshold with anticorrosive. We will also provide a comparative table of the cost of homemade and factory solutions so that you can evaluate the savings.

Why threshold amplifiers are needed: 3 key functions

Many car owners perceive threshold amplifiers as an exclusively “cosmetic” element, but in fact their role is much broader. Let's consider the main tasks that these details solve:

• 🛡️ Corrosion protection: The amplifier blocks the access of moisture and reagents to the internal cavities of the threshold, where rust spreads especially quickly. Even if the outer layer of paint is damaged, the metal of the amplifier remains intact.
• 💥 Increased passive safety: In the event of a side impact, the amplifier distributes the load, preventing deformation of the interior. In some models (for example, Volkswagen Passat B6) factory amplifiers are integrated into the energy absorption system.
• 🔧 Simplifying repairs: If the threshold is already rotten, the amplifier serves as a “framework” to restore the geometry of the body. This is cheaper than completely replacing the threshold by welding.

Interesting fact: on premium cars (for example, Mercedes-Benz S-Class or BMW 7 Series) Threshold reinforcements are often made from aluminum alloys or even carbon fiber to reduce weight without sacrificing strength. In home-made structures, such materials are rarely used due to the complexity of processing, but we will talk about this below.

⚠️ Attention: Threshold enhancers are not a complete replacement for repairing rotten metal. If the threshold is rusted through or deformed after an accident, you first need to restore its geometry (for example, using straightening or welding patches), and only then install the amplifier.

Materials for production: what to choose and why

Not only the strength of the amplifier, but also its corrosion resistance depends on the quality of the material. Let's look at the main options used by car enthusiasts and professionals:

Material	Thickness, mm	Pros	Cons	Cost per 1 m²
Galvanized steel	0.8–1.2	High corrosion resistance, strength	Difficult to weld without special equipment	800–1200 ₽
Cold rolled steel	1.0–1.5	Easy to bend, welds well	Rusting without protection	500–900 ₽
Aluminum (alloy 5052)	1.5–2.0	Lightweight, does not rust	Expensive, requires argon welding	2500–3500 ₽
Stainless steel	0.8–1.0	Does not rust, durable	Difficult to bend, high price	1800–2500 ₽

For most DIY projects, the best choice is galvanized steel 1–1.2 mm thick. It combines strength, corrosion resistance and a relatively low price. If you plan to strengthen the sills on an SUV (for example, UAZ Patriot or Nissan Patrol), you can take thicker steel - up to 1.5 mm, but keep in mind that this will complicate the bending process.

Aluminum and stainless steel are suitable for experienced craftsmen, as they require special equipment (argon welding) and skills in working with these materials. For example, when welding aluminum, it is necessary to use a filler wire made of an alloy ER5356 and protection with argon, otherwise the seam will be fragile.

Tools and equipment: minimum set

To manufacture and install threshold amplifiers you will need the following tool:

• 🔨 Bulgarian with cutting and grinding wheels (for example, Interskol UShM-125/1100E).
• 🔥 Welding machine: an inverter is suitable for steel (for example, Resanta SAI-190), for aluminum - argon arc welding.
• 📏 Ruler, square, marker for marking.
• 🛠️ Metal scissors or guillotine for cutting workpieces.
• 🔧 Drill with a set of drills (for fastening amplifiers with rivets, if you do not plan to weld).
• 🧴 Anti-corrosion compounds: ML-mastic, Zinkor or Dinitrol 4941.

If you don't have a welding machine, the amplifiers can be mounted on rivets (for example, aluminum or steel with anti-corrosion coating). However, this method is less reliable, since it does not ensure the tightness of the seam. To seal joints, use adhesive sealant for body (for example, 3M 08597).

⚠️ Attention: When working with an angle grinder and a welding machine, be sure to use safety glasses (for example, 3M Speedglas 9100) and respirator. Dust from grinding galvanized steel contains zinc particles, which when inhaled cause “foundry fever” - acute intoxication.

Step-by-step instructions: from pattern to installation

The manufacturing process of threshold amplifiers can be divided into 5 stages. We will consider each of them in detail, taking into account the nuances for different body types (sedan, hatchback, SUV).

1. Removing the pattern

The first step is to create an amplifier template that follows the shape of your car's threshold. To do this:

1. Clean the threshold from dirt and rust (you can use sandblaster or a wire brush).
2. Stick on the threshold masking tape and trace the outline with a marker.
3. Drag the outline to cardboard or thick paper, cut out the pattern.
4. Check that the pattern fits snugly against the threshold, adjust if necessary with scissors.

For symmetrical thresholds (for example, at Toyota Corolla or Hyundai Solaris) you can make one pattern and turn it around in a mirror for the second side. For asymmetrical bodies (eg Nissan Juke) you will have to remove the patterns separately for the left and right thresholds.

2. Cutting blanks

Transfer the pattern to the metal and cut out the blanks with an allowance of 5–10 mm along the edges (for fitting). For cutting you can use:

• 🔪 Metal scissors — for thin steel (up to 1 mm).
• ⚡ Plasma cutter — for thick metal (from 1.5 mm).
• 🔄 Laser cutting — if you order blanks from a workshop (more accurate, but more expensive).

After cutting, process the edges of the workpieces sandpaper P80 or grinding wheelto remove burrs. If the amplifier will be attached by welding, make it on the workpiece flange (curved edge) around the perimeter - this will make it easier to tack to the threshold.

3. Fitting and pre-installation

Place the workpiece against the threshold and check the fit. Please note:

• 📍 Mounting holes for bolts or rivets (if you don’t plan to weld).
• 🔄 Bends: if the threshold has a complex shape (for example, on Mazda CX-5), you may have to bend the workpiece to sheet bender or manually using a hammer and mandrel.
• 🚫 Crossing points with other body elements (for example, with jacks or floor reinforcers).

If the amplifier will be welded, do potholders (short welds) in 3-4 places, then check the geometry. If everything is fine, weld the seam completely. For galvanized steel, use rutile-coated electrodes (e.g. ESAB OK 46.00).

4. Welding and sealing

When welding amplifiers, observe the following rules:

• 🔥 Welding current: for steel 1 mm - 60–80 A, for 1.5 mm - 100–120 A.
• ⚡ Polarity: reverse (electrode on “+”, part on “−”) for better penetration.
• 🛡️ Protection: After welding, clean the seam metal brush and cover zinc-containing primer (for example, Body 960).

If the amplifier is mounted with rivets:

1. Drill holes in the threshold and reinforcement (diameter 0.1 mm less than the diameter of the rivet).
2. Apply to joints seam sealant (for example, Teroson Terostat 9330).
3. Install the rivets using riveter (for example, Kraftool 2451-10).

5. Anti-corrosion treatment

After installing the amplifier, be sure to treat the internal cavities of the threshold. To do this:

1. Drill into the threshold technological holes (diameter 8–10 mm) for access to cavities.
2. Apply anticorrosive with the help sprayer with flexible nozzle (for example, ML-mastic or Waxoyl).
3. Close the holes rubber plugs or brew it.

For external protection cover the amplifier primer for metal (for example, PPG DP40) and paint it in body color. If the amplifier will be visible (for example, on SUVs), you can use powder paint - it is more wear-resistant.

What to do if the threshold is already rotten?

If the threshold is rusty through, first cut out the rotten areas with a grinder, then weld patches of steel of the same thickness. Only after the body geometry has been restored can the amplifier be installed. For precise cutting, use a cardboard template transferred to metal.

Common mistakes and how to avoid them

Even experienced craftsmen sometimes make mistakes when making threshold amplifiers. Here are the most common ones and ways to prevent them:

• 🔥 Overheating of metal during welding → leads to deformation of the threshold. Solution: cook in short seams (2-3 cm each) with breaks for cooling.
• 💧 No drainage holes → water accumulates inside, accelerating corrosion. Solution: Drill holes with a diameter of 5–6 mm at the bottom of the threshold.
• 🧲 Use of magnetic soils on aluminum amplifiers → the soil does not adhere. Solution: use epoxy primer (for example, PPG K36).
• 🔩 Rivets without sealing → moisture penetrates the joints. Solution: apply seam sealant before installing rivets.

Another common mistake is ignoring body geometry after installing amplifiers. For example, if the threshold is “swollen” from welding, this can lead to problems with opening doors. To avoid this, after welding, check the gaps between the door and the threshold (they should be the same along the entire length).

Comparison of homemade and factory amplifiers

To understand whether it is worth making amplifiers yourself, let’s compare them with factory solutions based on key parameters:

Parameter	Homemade amplifiers	Factory Amplifiers
Cost	1500–3000 ₽ (materials + anticorrosive)	3000–15,000 ₽ (depending on the car model)
Strength	Depends on the material and quality of welding	Guaranteed strength (tested by manufacturer)
Corrosion resistance	Requires careful anticorrosive treatment	Galvanized or factory coated
Difficulty of installation	Requires welding and straightening skills	Easier (often comes with a mounting kit)
Individuality	Can be adapted to any body	Only suitable for specific models

Homemade amplifiers are beneficial if:

• 🚗 You have unique or rare model a car for which there are no factory amplifiers.
• 💰 Do you want save money (the difference in price can reach 10,000 ₽).
• 🛠️ Do you have tool and skills working with metal.

Factory amplifiers are worth choosing if:

• ⏱️ Important to you installation speed (no need to customize to size).
• 🔒 Do you want guarantee on material and corrosion protection.
• 🔧 You have no welding experience (factory amplifiers are often mounted with bolts or rivets).

Additional ways to protect thresholds

Amplifiers are only part of comprehensive threshold protection. To maximize the life of the body, use the following methods:

• 🛡️ Anti-gravel film: glued to the outer surface of the threshold (for example, 3M Scotchgard). Protects against chips and scratches.
• 🧴 Liquid locker: applied to the internal cavities of the threshold (for example, Noxudol 700). Creates an elastic coating that does not crack when deformed.
• 🔧 Plastic linings: installed on the thresholds of SUVs (for example, on Toyota Land Cruiser 200). Protects against mechanical damage when driving off-road.
• 🔥 Galvanizing: galvanic or cold (for example, spray Zinc Rich Primer). Increases the corrosion resistance of metal.

For vehicles operated in aggressive conditions (for example, in coastal cities or on winter roads with reagents), it is recommended to combine several methods. For example:

1. Install threshold amplifiers (protection from the inside).
2. Treat cavities liquid locker.
3. Paste anti-gravel film outside.
4. Wash thresholds regularly (every 2–3 months) special shampoos (for example, Karcher RM 539).

Remember that even the most reliable amplifiers will not save thresholds if you ignore regular care. For example, after driving through mud or salt, be sure to wash the sills jet of water under pressure (you can use a mini-wash Karcher K2).

FAQ: Frequently asked questions about threshold amplifiers

Is it possible to make threshold amplifiers without welding?

Yes, amplifiers can be mounted on rivets or bolts, but this method is less reliable. To seal joints, be sure to use adhesive sealant for body (for example, 3M 08597). You can also combine rivets with structural adhesive (for example, SikaPower-498), which provides strength comparable to welding.

Which metal is better: galvanized or stainless steel?

Galvanized steel is cheaper and easier to weld, but over time, the zinc coating in the areas of cuts and welds loses its protection. Stainless steel does not rust, but is more difficult to bend and weld (argon welding is required). For most projects, galvanizing with a thickness of 1–1.2 mm with additional processing is optimal zinc-containing primer after welding.

Do I need to paint the outside of amplifiers?

If the amplifiers are not visible (for example, hidden by plastic covers), it is enough to process them anticorrosive and soil. If the amplifiers are exposed (for example, on SUVs), it is better to paint them in body color or cover them powder paint for added protection. Use epoxy primer before painting - it improves adhesion.

How long will homemade amplifiers last?

The service life depends on the material and the quality of anti-corrosion treatment. With proper installation and regular maintenance, homemade galvanized steel amplifiers serve 5–7 years, stainless steel - 10+ years. The main thing is not to forget to treat joints and internal cavities with anti-corrosion agent once every 1-2 years.

Is it possible to install amplifiers on a rusty threshold?

No, first you need to remove rust and restore the metal. If the threshold is rotten through, cut out the damaged area and weld a patch of steel of the same thickness. Only after this can the amplifier be installed. Ignoring this rule will lead to the fact that rust will continue to spread under the amplifier, and after 1-2 years the threshold will have to be changed completely.