Almost never — and certainly not as a first step.
Bobbin tension is set at the factory and drifts very slowly if at all. The overwhelming majority of faults people try to fix with the bobbin screw are top-thread faults: threading, needle, or lint.
Work the real list first. Re-thread with the foot up. Change the needle. Check the bobbin class and that it is inserted turning the right way. Clean the race. Adjust top tension in half-numbers. Between them these fix nearly everything.
If you must adjust it. Note the starting position first — mark the screw head with a pen so you can return to it. Turn in very small increments, no more than a sixteenth of a turn at a time, and test after each. Clockwise tightens.
The drop test, if your machine has a removable bobbin case: hold the threaded case by the thread and let it hang. It should slip down slowly when you jerk your hand gently, and hold when still. That is roughly correct tension.
Buy a second bobbin case if you regularly sew with very different thread weights. Adjust that one and leave the original alone. It costs far less than a service and it means you always have a known-good baseline to return to.
Questions people actually ask
My bobbin case has no screw.
Many drop-in bobbin systems are not user-adjustable at all. That is deliberate, and it means the answer to your fault is somewhere else.
A video told me to adjust it.
A great deal of online advice reaches for the bobbin screw early. It is the last resort in every manual we have read, and there is a reason for that.
Last reviewed 2026-08-11