Because lint packed between the discs holds them apart, so they can never close on the thread properly — and the dial stops doing anything useful.
How it gets there. Every metre of thread carries a small amount of fibre and finish. Over months it collects at the narrow point where the thread passes between the discs, and it compacts into a felt. Fluffy fabrics and poor thread accelerate it considerably.
The symptom. Top tension that will not go tight enough at any setting. Loops on the underside that persist after a correct rethread. A machine that used to be fine and has drifted gradually, rather than failing suddenly.
How to clear it. Raise the presser foot, which opens the discs. Then run a folded strip of thin firm fabric, or a length of strong thread used like a shoeshine cloth, through the gap. Repeat until nothing comes out. Some machines let you turn the dial to its loosest setting to open the gap further.
What not to use. Anything metal, including pins and tweezers. A scratch on a tension disc is permanent and it will shred thread for the rest of the machine life. Compressed air is also poor practice, because it drives lint deeper into the machine rather than out of it.
How often. Once or twice a year for ordinary use, and more often after fleece, corduroy or towelling.
And clean the bobbin race at the same time, which is where the rest of the lint goes.
Questions people actually ask
Can I use a vacuum instead?
A small vacuum with a narrow nozzle is good for loose lint in the race. It will not reach compacted lint between the discs, which needs the fabric strip.
My machine is new and already doing this.
Then it is more likely a threading or needle fault. Compacted lint takes months to build up.
Last reviewed 2026-08-11