Have you ever stood in an auto parts or sewing supply store holding two labeled spools of thread? №40 and №50, and wondered: which of them are thicker? At first glance, it seems that the higher the number, the thicker the thread - but this is a common misconception. In fact, everything works exactly the opposite: than less number, the thread thicker.

In this article we will look at why this happens, what thread numbering systems exist (and why this is important for car owners), and also where threads are used №40 and №50 - from sewing interior covers to repairing a soft car roof. You will learn how not to overpay for unnecessary thickness and not to spoil the material with the wrong thread.

Why No. 40 is thicker than No. 50: understanding the numbering

All threads are marked according to two main systems: metric (European) and textile (American). In both there is an inverse relationship: the lower the number, the thicker the thread. This is due to the fact that the number does not indicate the diameter, but thread length per unit weight.

For example, in metric system (denoted as Nm or just a number):

  • 🧵 №40 - this is 40 kilometers of thread weighing 1 kg. The shorter the length per kilogram, the thicker the thread.
  • 🧵 №50 - this is 50 km per 1 kg. Longer length for the same weight means thinner thread.

B textile system (denoted as Ne or cotton count) the logic is the same: №40 thinner than №30, but thicker than №60. However, for auto repair, the metric system is more often used, especially when working with synthetic threads (polyester, nylon).

📊 Which threads do you use most often for cars?
Polyester
Cotton
Nylon
I don't know which ones are suitable

Comparison table: No. 40 vs No. 50 by key parameters

To clearly see the difference, we have collected the main characteristics of the threads №40 and №50 in the table. Pay attention to the "Recommended Application" column - it will help you choose threads for specific applications in the car.

Parameter Threads No. 40 Threads No. 50
Thickness (diameter) 0.25–0.30 mm 0.20–0.25 mm
Tensile strength High (up to 8 kg) Medium (up to 6 kg)
Material Polyester, nylon, cotton (thick) Polyester, cotton (standard), silk
Recommended Application
  • 🚗 Seat covers (leather, Alcantara)
  • 🚗 Repair of soft roofs (convertibles, awnings)
  • 🚗 Sewing bumper covers
  • 🧵 Lightweight fabrics (velor, suede)
  • 🧵 Decorative seams (headrests, handles)
  • 🧵 Door trim repair
Price (per 100 m) 120–250 ₽ 90–180 ₽

Important: when working with synthetic materials (for example, polyester covers) it is better to use threads №50 - they damage fabric fibers less. For genuine leather or thick vinyl will do №40.

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Thread #40 can withstand heavy loads, but can leave noticeable marks on thin fabrics. No. 50 is more universal for delicate materials.

Where in the car are threads No. 40 and No. 50 used?

Car owners often underestimate the role of threads in interior or body repairs. Meanwhile, the wrong choice can lead to:

  • ⚠️ Seam divergence on covers due to insufficient thread strength.
  • ⚠️ Damage to material (for example, punctures with thick thread No. 40 on thin Alcantara).
  • ⚠️ Corrosion (if you use cotton threads in places with high humidity).

Let's look at typical tasks and suitable threads:

🔧 For seat covers and upholstery

Materials: leather, artificial leather, Alcantara, velor.

  • 🛋️ No. 40 (polyester) — for dense materials (leather, vinyl). The strength of the seam is critical.
  • 🛋️ No. 50 (polyester or nylon) - for velor and thin Alcantara. Less damage to fibers.

🔧 For soft roofing and awnings

Materials: tarpaulin, PVC fabric, acrylic.

  • ⛺ No. 40 (nylon) — optimal for repairing truck or convertible awnings. UV and moisture resistant.
  • ⛺ No. 30–No. 40 (polyester with wax) - for reinforced seams (for example, fastening hatches).

🔧 For decorative elements

Materials: suede, fabric door trim, headrests.

  • 🎨 No. 50–No. 60 (silk or polyester) - for invisible seams.
  • 🎨 No. 40 (waxed polyester) - if additional strength is needed (for example, for door handles).
What happens if you use threads for other purposes?

Thick threads (no. 40) on thin fabric may cause material rupture when tensioning (for example, when installing seat covers). Thin threads (No. 60 and thinner) on dense materials (leather, tarpaulin) quickly will rub shoulders from friction, and the seam will come apart. This is especially critical for elements subject to load: handles, hinges, awning fastenings.

How to determine the thickness of a thread without markings?

Let's say you have a reel lying around in your garage without a label. How do you know if it's No. 40 or No. 50? Here are some practical ways:

  1. Comparison with a known sample: Take a new thread with markings and attach it to the unknown one. #40 will be noticeably thicker.
  2. Diameter measurement:
    • 📏 Use a caliper or micrometer. No. 40 — ~0.25–0.30 mm, No. 50 — ~0.20–0.25 mm.
    • 📏 Alternative: wrap 10 turns of thread around a pencil and measure the total width with a ruler. Divide by 10 to get the diameter.
  • Strength test: Pull the thread with your hands. No. 40 will break with a force of ~7–8 kg, No. 50 - with ~5–6 kg.
  • burning (for synthetic threads):
    • 🔥 Polyester melts with the smell of plastic, forming a hard ball.
    • 🔥 Nylon melts with the smell of hair, the ball is soft.
    • 🔥 Cotton burns like paper, without a ball.

    1. Determine the material (leather, fabric, PVC)

    2. Check the fabric manufacturer's recommendations

    3. Choose synthetic threads for wet areas

    4. Test the strength of the seam on scraps of material -->

    Top 5 mistakes when choosing threads for a car

    Even experienced craftsmen sometimes make mistakes with threads. Here are the most common mistakes and their consequences:

    ⚠️ Attention! Usage cotton threads to repair the soft roof of a convertible will lead to rotting of the seams after 1-2 seasons. Synthetics (polyester, nylon) are resistant to moisture and UV.
    • 🚫 Ignoring thread material. Cotton is cheaper, but not suitable for outdoor use (awnings, motorcycle covers).
    • 🚫 Saving on thickness. Thread #60 is cheaper, but for leather covers you need #40 - otherwise the seams will come apart.
    • 🚫 Ignoring color. Black threads on light velor will be visible. Use shades graphite gray or beige.
    • 🚫 Neglecting wax. Unwaxed threads fray faster in seams subject to friction (for example, on door handles).
    • 🚫 Storage under the sun. Polyester threads lose strength with prolonged UV exposure.

    Tip: When in doubt, buy a small reel №40 and №50 and test on scraps of your material. This way you can determine exactly which thread fits best.

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    To repair the car interior, use threads with polyurethane coating — they do not rub against the fabric and do not tear the fibers. For example, Gütermann Mara 70 (analogue No. 50) or Amann Seracor (№40).

    Which threads to choose for specific car brands?

    Car manufacturers often use specific materials for the interior, which require a special approach to the selection of threads. Here are some examples:

    Car make Typical interior material Recommended threads
    Mercedes-Benz (Class E/S) Genuine leather, Alcantara No. 40 (polyester Gütermann Serafil)
    BMW (episodes 3/5) Perforated leather, velor No. 50 (nylon Amann Mako)
    Toyota (Land Cruiser, Hilux) Thick fabric, vinyl No. 40 (waxed polyester Coats Epic)
    Volkswagen (Golf, Passat) Fabric covering, artificial leather No. 50 (polyester Madeira Polyneon)

    For convertibles (for example, BMW Z4, Porsche 911 Cabriolet) it is critical to use threads with UV stabilization, for example, Gütermann Solar (No. 40). They do not fade or lose strength under the sun.

    FAQ: Frequently asked questions about threads No. 40 and No. 50

    Can I use #40 thread instead of #50 thread for Alcantara covers?

    Not recommended. Thread #40 is too thick for Alcantara - it can cut through the fibers or leave noticeable marks. Optimal choice: No.50 or No.60 made of silicone-coated polyester (e.g. Amann Seracor).

    Which needle should I use for threads #40 and #50?

    For threads №40 a needle will do 90/14 (for skin) or 100/16 (for dense fabrics). For №50 — 80/12 (universal) or 75/11 (for thin materials). For auto repair, it is better to take needles from reinforced point (for example, Schmetz Leather).

    How are polyester threads different from nylon threads?

    Polyester (PES) is more resistant to UV and chemical influences, suitable for outdoor work (awnings, motorcycle covers). Nylon (PA) is more elastic and tensile strength, but is afraid of the sun and moisture - better for interior lining. Optimal for most tasks in the car polyester.

    How much #40 thread is needed to sew front seat covers?

    On average it takes one case 50–70 meters threads (taking into account double seams and reserve). For two cases, buy a reel 200–250 m. If you sew from leather, take extra thread - thick materials require more frequent stitches.

    Is it possible to dye threads for a car?

    Yes, but only synthetic (polyester, nylon) and special fabric paints (for example, Angelus Leather Paint). It is not recommended to dye cotton threads - they lose strength. Before dyeing, test the thread on a small section!