The question of what the real thickness of thread number 40 is often arises not only among professional seamstresses, but also among car service technicians who reupholster interiors, repair upholstery, or even do specific wiring work. In the automotive industry, the accuracy of the choice of materials is critical: a thread that is too thin will not withstand vibrations, and a thread that is too thick can damage the structure of the material or simply not fit into the eye of an industrial machine needle. Understanding the metric numbering system allows you to avoid defects and ensure the durability of the completed connection.
The numerical designation that we see on the reel is not just a random set of numbers, but a strictly regulated standard that determines the linear density and actual diameter of the fiber. For a thread marked 40, this parameter indicates the average thickness class, which is the universal “gold standard” for many tasks. However, in order to fully understand the issue, it is necessary to take into account the material of manufacture, since synthetics and natural fibers with the same number may have differences in texture.
In this article we will analyze in detail the physical thickness of thread No. 40 in millimeters, consider its use in the context of automotive repair and discuss the nuances of choice for various types of work. You will find out why this particular number is most often recommended for sewing covers and repairing upholstery, as well as what nuances exist when working with it in a garage or specialized workshop.
Metric numbering system and physical thickness
To understand how thick thread number 40 is, you need to refer to the metric numbering system, which is most common in our country and Europe. In this system, the thread number corresponds to the number of meters of thread contained in one gram of its weight. Therefore, thread No. 40 means that one gram of such material contains 40 meters of length. The higher the number, the thinner the thread, and vice versa - this is a basic rule that should be remembered when selecting consumables.
If we translate these abstract meters into specific physical dimensions, then the diameter of the 40th thread usually varies in the range 0.20–0.25 millimeters. This figure may vary slightly depending on the twist density and the type of raw material. For automotive craftsmen, this information is important when selecting needles: for this thickness, needles marked from 90 to 110 according to the European system (NM 90–110) are optimally suited, which ensures free passage of the thread without breaking the fabric fibers.
It is worth noting that the actual diameter depends on the number of folds. Often #40 thread is made up of several thinner fibers twisted together. If we are talking about a classic three-strand twist, then the total thickness gives the aforementioned range. In the automotive industry, where seam stress is high (especially in seats), not only thickness is important, but also the tensile strength that this configuration provides.
⚠️ Attention: When purchasing threads, pay attention to the country of origin. Chinese analogues labeled 40LX may have a real diameter that differs from the declared one by 10-15%, which will lead to frequent breaks or skipped stitches on high-speed machines.
Manufacturing materials: polyester, cotton and mixtures thereof
Thickness is not the only parameter influencing the choice. For automotive related work, the thread material is critical. The most popular option for number 40 is polyester thread (lavsan). It has high strength, resistance to rotting, moisture and ultraviolet radiation. In the conditions of a car interior, where the temperature can fluctuate and the sun's rays fall on the seats, synthetics show themselves to be much better than their natural counterparts.
Count 40 cotton threads also exist and are often used for temporary basting or working with natural materials where a breathable seam is required. However, they are less suitable for major repairs of car upholstery due to their hygroscopicity: absorbing moisture, cotton can rot and lose strength. Blends (such as cotton and polyester) are less common in the 40-count format for heavy duty work, but can be used in decorative stitching.
An important aspect is twisting and impregnation. High-quality 40-gauge automotive thread has a tight twist and is often coated with silicone or a special wax. This reduces friction as it passes through the material and needle, preventing overheating that could melt the synthetic fiber. The use of unimpregnated thread in high-speed machines for sewing covers is unacceptable.
- 🧵 Polyester (Lavsan): High strength, tensile strength, ideal for covers and seat belts.
- 🌿 Cotton: Soft, lacks shine, suitable for retro restoration, but is susceptible to moisture.
- ⚖️ Blended: A compromise between the elasticity of cotton and the strength of synthetics, it is rarely used for power seams.
Effect of twist on thickness
The thread can be single, double or triple twist. A triple twist (3-ply) with the same number 40 will give a thicker and stronger seam than a double one. For leather in cars, a tight twist is recommended.
Application of thread No. 40 in the automotive industry and repair
In the field of car maintenance and repair, thread number 40 has found wide application. This is the main working “tool” for reupholstering interiors. It is used to make seat covers and restore the soft upholstery of doors, ceilings and side panels. Its thickness allows you to make neat, invisible seams, which at the same time withstand the constant mechanical loads that arise when boarding and disembarking passengers.
In addition to upholstery, this thread is used for hemming rugs, repairing headrests, and even for fixing some sound insulation elements if the installation technology requires stitching. Unlike finer numbers (50, 60), thread 40 is less prone to breaking under strong tension, and unlike coarse numbers (20, 30), it does not leave wide holes in the fabric, maintaining tightness and aesthetic appearance.
Particular attention should be paid to decorative stitching. For contrast stitching on leather or Alcantara, number 40 is often used, as it fits well into the seam groove and creates an expressive line. However, here it is important to choose the correct foot pitch and upper thread tension so that the seam does not pull the material.
There is a misconception that leather only needs a thick thread. In practice, for medium-density automotive leather (1.2–1.4 mm), number 40 is the optimal choice. Thicker threads may require cutting the skin or using special needles with a wedge-shaped tip, which is not always justified.
Comparison table: Thread number and characteristics
To better understand the place of thread number 40 in the general classification, it is useful to compare it with neighboring values. This will help you make your choice if 40 seems too thin or, conversely, too rough for a specific task. The table shows average data, as manufacturers may have their own tolerances.
| Thread number (Metres) | Approximate thickness (mm) | Recommended needle size | Main application in cars |
|---|---|---|---|
| 30 | 0.30 – 0.35 | NM 110–130 | Rough leather, belts, awnings |
| 40 | 0.20 – 0.25 | NM 90–110 | Covers, upholstery, leather |
| 50 | 0.15 – 0.20 | NM 75–90 | Thin fabrics, lining, velor |
| 60 | 0.10 – 0.15 | NM 60–75 | Basting, fine silk, hem |
The table shows that thread No. 40 is in the middle, which makes it universal. Switching to number 30 is justified only when working with very thick materials, for example, when restoring cargo seats or sewing awnings. Upgrading to number 50 or 60 is necessary for delicate fabrics such as thin Alcantara or textile inserts in premium door panels.
⚠️ Warning: Do not attempt to insert #30 thread into a machine set to #40 without changing the needle and adjusting the tension. This will lead to needle breakage, thread breakage and possible damage to the shuttle mechanism.
Selection of needles and equipment settings
The success of working with thread number 40 directly depends on the correct selection of the needle. As mentioned earlier, the optimal range is needles NM 90, NM 100 and NM 110. The choice of a specific number depends on the density of the material. For thin eco-leather or fabric it is better to take 90, for medium-thick natural leather - 100, and for multi-layer compounds - 110.
It is important to use the correct type of needles. For skin these are needles with a point type LR (Leather Point) or LL, which cut the fibers rather than pushing them apart, as do fabric needles (type H). Using a fabric needle to sew leather with thread number 40 will cause the needle to become dull after the first stitches and deteriorate the quality of the seam.
☑️ Checking machine settings
The thread tension also requires adjustment. Thread #40, being synthetic, has minimal stretch. If the tension is too strong, the seam may “wave” or pull the material together. If it is too weak, the loops will remain at the bottom. An ideal seam should have a knot connecting the top and bottom threads within the thickness of the material.
Tips for storage and use
Even the highest quality thread can deteriorate if stored improperly. Number 40 polyester threads are sensitive to direct ultraviolet radiation. Exposing spools to the sun for a long time (for example, in a store window or on a table by a window in a garage) causes the polymer to degrade and the thread becomes brittle. They should be stored in closed cabinets or boxes.
Moisture is another enemy, although less so for synthetics. However, high humidity combined with dust can cause a coating to form on the thread, which will clog the eye of the needle and the tension mechanism during sewing. In a garage where painting or welding work is often carried out, it is better to keep the coils in sealed containers.
Before starting to work with a new spool of #40 thread, especially if it has been lying around for a long time, make a few meters of a test stitch. This will allow the thread to straighten and warm up, which will stabilize the tension at the beginning of the main seam.
When working with high-speed industrial machines, it is important to pay attention to the winding direction. The thread should come off the spool freely, without jerking. If using a vertical spool holder, make sure that the thread does not twist around its axis, as this creates additional knots and bulges that can get stuck in a needle with a diameter of only 0.25 mm.
Proper storage of threads away from sunlight and aggressive chemicals extends their service life and guarantees the strength of the seam for years to come.
Frequently asked questions (FAQ)
Can I use 40 thread to sew together seat belt pieces?
Absolutely not. Repairing seat belts requires the use of special high-strength thread (usually 20-30 or specialized nylon thread) and industrial equipment certified to ensure life safety. Thread No. 40 will not withstand the jerk load during an accident.
Is thread 40 suitable for handmade leather work in a car?
Yes, thread No. 40 is perfect for decorative work or minor hand repairs. It is strong enough not to tear when the knot is tightened, and thin enough to fit through punctures in the skin. For hand stitching, it is better to use the waxed version of the thread.
What is the difference between 40LL thread and 45LL thread?
Thread 45LL (lavsan lavsan) is slightly thinner than 40LL. The difference in diameter is approximately 0.03-0.05 mm. Visually, it is almost invisible on the seam, but number 45 may tear more often when sewing thick layers or on old machines with poor synchronization.
What stitch length should I use for thread number 40?
The optimal stitch length for thread No. 40 when working with automotive materials is from 3.5 to 4.5 mm. A finer stitch (2.5-3mm) is used for fine fabrics or decorative stitching, but it can perforate the leather, weakening it.
Is it possible to melt the end of a #40 thread?
Yes, since most #40 auto thread is synthetic (polyester), the end can be carefully melted with a lighter to prevent the knot from fraying. However, this must be done carefully so that a large melted ball does not form that will not pass through the material when the seam begins.