Don't use crummy thread. I use 100 percent polyester for everything. I buy only Gutermann -- and it is readily available.
The cotton coated polyester stuff you see in the various local sewing shops serves no purpose for the home sewer. Cotton coated polyester was developed to be used in high speed commercial machines that are sewing so fast and continuously that the needle gets hot and melts the polyester when the machine stops briefly. This is hardly what you need for home sewing and my impression is that it must be "surplus" from the commercial market.
Mara and Tera seem to be Gutermann specific terms or trade names. I have a bit of what is called "Mara 100" and it is Tex 30. I have a lot of "Mara 70" and it is Tex 40. Mara 50 is even stouter (it is Tex 60), but I don't have any of that.
I do have one cone of "Tera 80" which is Tex 35.
I have another cone of "Tera 60" which is Tex 50.
See below on Tera 40, which is Tex 75.
For heavy duty use I would call Tera 60 the best choice. Even though it is thinner than my Mara 70, it is probably stronger. Any machine should handle it, which is not the case with Tera 40.
I now have Tera 40 in hand, and can use that for really hard use cases, but see my notes below.
Here is some information from Gutermann:
Most fabric stores carry Gutermann Sew All which is a Mara thread and the most common size is Tex 30. Mara is a MicroCore Spun Polyester thread and Tex 30 is the most popular Tex size sold in the home-sewing market.
Mara is a MicroCore, Spun Polyester thread, made in many Tex sizes and is typically used for construction and decorative stitching on apparel, leather and accessories. The appearance of Mara is a low luster, matte finish with a 'natural' look of silk thread. The thinner Tex sizes are used for garment construction and the thicker Tex sizes are good for buttonholes, decorative topstitching (denim) and leather accent stitching on handbags, wallets, belts, etc.
Tera is a Continuous Filament polyester thread that is made in a different technique and is known for it silky, shiny surface, and strong resistance to abrasion. Tera is slightly stronger than Mara (pound for pound) in like Tex sizes and Tera is normally used for upholstery, leather stitching, outdoor fabrics (awnings, marine upholstery, umbrellas etc.
Both threads are made of 100% polyester, so they both have similar resistance to UV and chlorine, but normally Tera is more commonly used outside or in high sunlight exposure.
Sometimes, people use names that are confusing and normally, our Gutermann customers use the threads 'proper name as shown on the cone or spool. Some retailers use descriptions of the thread that do not match the thread name and this is a little misleading. The Sew All thread you see in Joanne stores or other retail fabric shops is Mara 100 (Tex 30). The heavy topstitch or heavy duty Mara is normally Mara 30 (Tex 100), typically used for denim. This is also called jeans topstitch thread. Tera is found in some retail stores under the name "Upholstery thread) and this is Tera 40 (Tex 75).
Most Distributors use the actual thread name like Mara or Tera and use the corresponding Tex size to describe the weight or thickness of the thread. For hammocks, backpacks or similar applications, the Tera 40 is my recommendation. It is available in most of our Distributors and the color range is fairly wide. The thinner Tera threads are great for ripstop nylon as this fabric has a tendency to pucker - so the thinner the thread, the smaller the needle and the least amount of pucker. In that case, Tera 80 (Tex 35) is perfect. This thread is not as widely distributed as Tera 40, but should be carried by more Distributors in the future.
Weight is the inverse of Tex. W = 1000 / Tex or Tex = 1000 / w. So bigger numbers indicate a finer thread. The defining idea is that "weight" is the number of kilometers of thread required to weigh 1 kilogram.
Denier is the weight in grams of 9000 meters of thread. D = 9 * tex and Tex = D / 9. Like Tex, bigger numbers indicate a stouter thread.
Mara; Tex; Needle 220; 13; 120; 25; 100; 30; "10-12" (sew-all) 70; 40; "14-16" 30; 100; TERA; TEX; Needle 80; 35; "10-14" 60; 50; "14-16" 40; 75; "16-18" 30; 100; 20; 150; 15; 200; 10; 290; 8; 375;
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