SINGER’s support page titled for the Heavy Duty 6800C lists examples for a “SINGER Heavy Duty machine,” but does not enumerate every model covered by that general wording. Use the examples below as a guide to that support page, not proof that every 6800C or other Heavy Duty model accepts every listed thread. Check the manual for your exact machine and market; do not transfer these limits to the SINGER 4423.
| Support-page examples | Stated guidance |
|---|---|
| All-purpose, 100% polyester, or 100% cotton | Listed examples for a SINGER Heavy Duty machine |
| Machine-quilting, 30 wt or 12 wt cotton, topstitch, or jeans-topstitch thread | Listed examples; confirm the exact machine and project setup |
| Bonded nylon or polyester | No larger than #69/Tex 70 in the support-page examples |
| Larger than #69/Tex 70, waxed or coated thread, or hand-quilting thread | Not recommended by the support page |
The page says some heavier needle threads may need all-purpose or polyester thread in the bobbin. A thread that is too thick for the bobbin may not feed correctly. It also advises using a needle whose eye lets the thread pass freely. These checks are separate: the thread must suit the machine, the bobbin system, the needle eye, and the fabric and stitch.
For most regular sewing thread, the same SINGER page describes dial settings 3–5 as normal; it does not make that range a specialty-thread setting or a guarantee for every fabric. If thread shreds, jams, or stitches become uneven, check threading, needle condition, bobbin seating, and the manual before changing tension. The bonded-thread guide explains why a supplier’s needle suggestion does not establish machine compatibility.