Only after a test line tells you to — but a change of thread is one of the three things that genuinely moves tension. The others are the fabric and the needle.
A different colour of the same thread changes nothing mechanically. Some dyes and finishes differ slightly, but not enough to matter for construction sewing.
A different weight does. A thicker thread fills the tension discs and the needle eye differently, so the same dial number holds it back harder. Move to a heavy topstitch thread and you will usually need to loosen the top tension, sometimes considerably.
A different fibre does. Cotton has almost no give, polyester has some. Under the same tension a polyester thread stretches slightly and a cotton one does not, which changes how the stitch locks and how much the seam puckers.
Metallic and rayon are the awkward ones. Both are weak in tension and both want a looser setting than you expect, along with a slower speed.
Mixing threads is what really causes trouble. Cotton on top and polyester in the bobbin behave differently under load, and no dial setting balances them. Match the fibre top and bobbin for ordinary sewing.
The practical routine. New spool, sew a test line, look at both sides, adjust if needed. It takes a minute and it prevents an evening of unpicking.
And do not chase small differences. If the stitch looks balanced and the seam is strong, the setting is right.
Questions people actually ask
Do I retest for a new spool of the same thread?
Not normally. Do it if the spool comes from a different brand, because thread weights are not standardised between manufacturers.
What about the bobbin thread?
It matters as much as the top thread. A finer bobbin thread changes the balance even when the top thread is unchanged.
Last reviewed 2026-08-11