Often slightly looser than normal, because a single bobbin thread has to lock with two top threads at once.
How a twin needle works. Two needles on one shank share the bobbin thread, which zigzags between them underneath. That thread is doing twice the work, and at a normal top tension it can be pulled too hard, lifting the fabric between the two lines into a ridge.
When the ridge is wanted and when it is not. On a knit hem a slight raised channel between the lines is the usual look. A hard ridge that will not press flat is too much tension.
What to change. Loosen the top tension by half a number and test. On very stretchy fabric some people also use woolly nylon in the bobbin, wound loosely by hand, which gives the seam some give.
Other settings that matter more than you expect. Slow right down — twin needles break easily. Check the needle plate hole is wide enough for the width you have chosen, by turning the handwheel a full cycle by hand before pressing the pedal. And never select a stitch wider than the needle spacing allows.
A skipped stitch on one line only is usually the wrong needle type rather than tension. On knits use a twin stretch needle, not a twin universal.
And the two top threads should feed separately, one from each spool pin, so they do not twist together.
Questions people actually ask
Which spool pin do I use?
Both. Most machines have a second pin, or a hole for an accessory one. Feed the threads so they do not wrap around each other on the way down.
My twin needle hem tunnels badly.
Loosen the top tension, and stabilise the hem with a strip of fusible tape. Tunnelling on knits is a stability problem as much as a tension one.
Last reviewed 2026-08-11