When it comes to car upholstery repair, upholstery, or even sewing seat covers, choosing the right thread becomes critical. Many car owners and craftsmen are faced with confusion in labeling: what do the numbers mean? 40 and 50 on the package, and which thread is actually thicker? At first glance, it seems that the logic should be reversed - the higher the number, the thinner the thread, but is this true in practice?
In this article, we will understand not only how to correctly interpret thread numbers, but also why this parameter directly affects the strength of seams in a car interior. You will learn what marking standards exist, how to convert the metric system into the usual millimeters, and which threads are best to choose for working with leather, Alcantara or fabric upholstery. We will also reveal one little-known nuance: why manufacturers of auto leather products often prefer threads with the number 50, despite their “thinness”.
Metric vs. textile system: why the numbers are misleading
The main reason for the confusion lies in the fact that there are two main thread marking systems: metric (European) and textile (American). In the metric system, which is most often used in Russia and Europe, the thread number designates it linear density - that is, how many meters of thread weighs 1 gram. Accordingly:
- 🧵 Thread No. 40 - 40 meters weigh 1 gram (thinner, since the same weight accounts for more length).
- 🧵 Thread No. 50 - 50 meters weigh 1 gram (even thinner, since the length has increased with the same weight).
But there is a catch: in the textile system (used in the USA and some Asian countries) the logic is reversed! There the number means weight threads to a fixed length. That is, the higher the number, the thread thicker. This often confuses craftsmen working with imported materials.
For automotive repairs, it is critical to understand that most European and Russian thread manufacturers (for example, Gütermann, Madeira, Amitex) use metric system. This means that thread No. 50 will be thinner than No. 40. But how does this affect the strength of the seam?
Thread thickness in millimeters: how to convert the number into real dimensions
To finally clarify the situation, let's convert abstract numbers into familiar millimeters. The thickness of the thread depends not only on its number, but also on the material (polyester, nylon, cotton). However, to simplify things, you can use approximate ratios:
| Thread number (metric) | Approximate diameter, mm | Typical application in auto repair |
|---|---|---|
| №30 | 0.30–0.35 | Heavy materials: 3+ mm thick leather, seat belts |
| №40 | 0.20–0.25 | Standard automotive leather (1.5–2.5 mm), Alcantara, thick fabrics |
| №50 | 0.15–0.20 | Thin leather, velor, decorative seams, headrests |
| №60 | 0.10–0.15 | Overlock, hemming, lightweight fabrics (steering wheel covers) |
It is important to note that the diameter may vary depending on twists thread (number of turns per inch). For example, thread Gütermann Mara 70 at number 70 it has a diameter of about 0.12 mm, but due to its high twist it can withstand a load comparable to thread number 50 from other manufacturers.
Threads are most often used for automotive work №40–№50. But why do craftsmen sometimes choose a thinner thread No. 50 instead of No. 40 if it seems less durable? The answer lies in the characteristics of the materials and sewing techniques.
When working with leather thicker than 2 mm, use a thread a size thinner than recommended - this will reduce the risk of the needle cutting through the material at high sewing speeds.
Seam strength vs. thread thickness: what is more important for car upholstery?
At first glance, it may seem that thick #40 thread will provide a stronger seam. However, in reality everything is more complicated. The strength of a seam depends on three key factors:
- Thread material: polyester threads (for example, Madeira Polyneon) are stronger than cotton ones at the same thickness.
- Twist: high twist threads (e.g. Serger) can withstand heavy loads.
- Needle and thread matching: If the needle is too thick for the thread, it will tear the fibers.
Often used in automotive repair polyester thread No. 50 for the following reasons:
- 🔹 Damages the skin less when passing through the material (especially relevant for thin natural leather).
- 🔹 Provides a neater seam with minimal “pulling” of the fabric.
- 🔹 Modern polyester threads No. 50 are not inferior in strength to cotton threads No. 40.
However, there are exceptions. For example, when repairing seat belts or sewing covers for trucks, preference is given to threads №30–№40 due to increased loads. It is also worth considering that some types alcantara require thread No. 60 to avoid “cutting” through the material.
Why do high-end car dealerships use silicone-coated threads?
Silicone coated threads (e.g. Gütermann Serafil) reduce friction when sewing, which is especially important for automated sewing machines. They are also abrasion and UV resistant, extending the life of your car's seams by 30-40%.
How to choose a thread for a specific task: a practical guide
To avoid making a mistake with your choice, use these instructions:
Determine the type of material (leather, Alcantara, fabric, velor)
Measure the thickness of the material with a caliper
Select a thread that is 10–20 units finer than recommended for the given thickness.
Check the compatibility of the thread with the needle (the needle diameter should be 0.05–0.1 mm larger than the thread diameter)
Make a test stitch on a piece of material -->
Examples of thread selection for popular tasks:
- 🚗 Leather steering wheel: thread №50–№60 (polyester, silicone coating).
- 🚗 Repairing worn seat upholstery: thread №40 (polyester, high twist).
- 🚗 Sewing trunk covers: thread №30–№40 (moisture resistant nylon).
- 🚗 Decorative seams on headrests: thread №60–№80 (silk or polyester with glitter).
Pay special attention to the color of the thread. For automotive work, threads with UV protectionso that the seams do not fade in the sun. Popular brands specializing in auto threads: Amann Group (series Elysee), Coats (line Epic), Amitex Profi.
For automotive leather repairs, the best choice is #50 polyester thread with a silicone coating. It combines strength, wear resistance and minimal damage to the material.
Common mistakes when choosing threads and how to avoid them
Even experienced professionals sometimes make mistakes that lead to disappointing results. Here are the most common of them:
⚠️ Attention: Using cotton threads for car upholstery is a big mistake. Cotton absorbs moisture, rots and loses strength after only 1–2 years of use. Only synthetic threads are suitable for cars!
- ❌ Ignoring thread twist: threads with low twist (for example, cheap "universal" threads) quickly fall apart into fibers.
- ❌ Needle and thread mismatch: thick needle + thin thread = torn fibers and weak seam.
- ❌ Saving on quality: cheap threads (for example, no-name Chinese ones) can contain up to 30% impurities that reduce strength.
- ❌ Neglecting the test seam: Even a perfectly selected thread can behave unpredictably with a particular material.
Another typical problem is improper thread storage. Polyester threads lose up to 20% of their strength when stored in the sun or in high humidity conditions. Optimal conditions: temperature 18–22°C, humidity not higher than 60%, in closed containers.
If you are working with genuine leather, pay attention to the threads with wax coating (for example, Fil au Chinois). They reduce friction when sewing and prevent the needle from “chewing” the leather.
Specialized threads for cars: when standard ones are not suitable
For some tasks, regular sewing threads are not suitable. Let's consider specialized options:
| Thread type | Application | Examples of brands |
|---|---|---|
| Threads with Kevlar core | Seat belts, child seat anchors | Coats Dual Duty Plus, Amann Bond-Tex |
| Threads with antibacterial coating | Medical cars, taxis | Gütermann Serafil Bio, Madeira Sanitized |
| Luminescent threads | Decorative seams in tuning, interior lighting | Madeira Lumina, Sulky Glow |
| High speed sewing threads | Industrial sewing of covers, conveyor repair | Amann Mako, Coats Epic Quilting |
For repair airbags threads are used special certification (for example, Amann Safety). They must withstand tensile loads of up to 50 kg and not melt at temperatures up to 200°C.
If you are doing restoration of vintage cars, pay attention to the threads with cotton core and silk covering (for example, Au Ver à Soie). They imitate historical finishes, but are reinforced with modern fibers.
Where to buy high-quality threads for car repairs: trusted suppliers
Not only the price, but also the originality of the product depends on the place of purchase. Here are reliable options:
- 🛒 Specialized auto stores: AutoUniverse, TD "Avtofabric" (Moscow, Yekaterinburg, Novosibirsk).
- 🛒 Online platforms: OZON (official stores Gütermann, Madeira), Wildberries (brand Amitex).
- 🛒 Wholesale suppliers: Sewing House (St. Petersburg), TextileProfi (Kazan).
- 🛒 Foreign sites: Amazon (search by
Upholstery Thread), AliExpress (stores with a rating above 98%).
When purchasing, pay attention to:
- 🔍 Packaging: original threads have holographic stickers (in Gütermann) or unique barcodes.
- 🔍 Color fastness: Rub the thread with a white cloth - if a mark remains, it is a fake.
- 🔍 Certificates: certificates are required for autothreads Oeko-Tex Standard 100 or ISO 9001.
⚠️ Attention: Counterfeit threads are common on the market Madeira and Gütermann, which are externally indistinguishable from the original, but break under a load of 5–7 kg. Check the supplier through the official websites of the brands!
Average prices for quality threads (2026):
- 💰 Gütermann Mara 70 (1000 m) - 800–1200 rub.
- 💰 Madeira Polyneon (500 m) - 600–900 rub.
- 💰 Amitex Profi (1000 m) - 400–700 rub.
- 💰 Coats Epic (special series for cars, 500 m) - 1200–1800 rubles.
FAQ: Answers to frequently asked questions about car repair threads
Can I use household thread (for example, from a sewing machine) to repair car upholstery?
No, household threads are not suitable. They cannot withstand vibrations, temperature changes and humidity inside the car. For car repairs, you need specialized threads with increased wear resistance (for example, silicone coated polyester). The exception is temporary repairs (for example, filing a cover for a day or two).
What thread color should I choose for an invisible seam on a leather seat?
For an invisible seam, use a thread 1-2 shades darker than the base color of the leather. For example, for black leather - dark gray thread (Gütermann 1040), for beige - light brown (Madeira 1520). A complete color match often looks unnatural. To check, make a test seam on the back of the seat.
What is the difference between threads for hand sewing and machine sewing in the context of auto repair?
Machine threads (eg Amann Mako) have a higher twist and are coated with a special lubricant to reduce friction in the machine mechanism. Hand sewing threads (eg. DMC) softer, but less durable. For auto repair use only machine threads, even if you sew by hand, they retain their strength longer.
How many threads do you need to completely reupholster the interior of a medium sedan?
For a complete reupholstery (seats, door panels, ceiling) you will need:
- 🧵 Main thread (No. 40–No. 50): 3–4 spools of 1000 m each.
- 🧵 Decorative thread (for contour seams): 1-2 spools of 500 m each.
- 🧵 Overlock thread (No. 60): 1 spool of 1000 m.
We recommend taking threads with a reserve of 20–30%, since when working with leather the consumption is higher due to the need for double seams at the joints.
Is it possible to dye threads with leather paint if there is no suitable color?
Technically possible, but not recommended. Leather paint (Angelus, Fiebing's) changes the structure of the thread, making it more fragile. If there is no other option:
- Use water-based acrylic paint.
- Apply 2-3 thin coats and allow to dry.
- After dyeing, treat the thread silicone grease to restore slip.
It is better to order the thread of the desired color from the manufacturer - many brands (for example, Gütermann) offer a custom coloring service.