Measure the gap instead of guessing it, then confirm the hook point passes the scarf as the needle rises. Gap, height and alignment, in check order.
Front-loading cases have a screw you can turn a quarter at a time; drop-in covers are stamped and fixed. Bending a fixed spring is how cases get replaced.
Raise the needle to full height, hold both threads back and seat the bobbin case the right way round; most pick-up failures end at one of those three.
Memory holds the stitch data and the motor control system replays it. What that buys over a cam stack, and where a stored pattern still limits a design.
Class 15 is the standard bobbin for both Singer and Brother, but the specification is loose. What actually decides whether one fits.
Turn the handwheel by hand first, then judge stitch quality and motor sound, and count the parts you already know it needs before you commit.
Fuzz in the discs, drifting tension and breaks at speed usually trace to the thread. What fibre and weight suit your fabric, and what cheap thread costs.
Most snap-on feet cross models, but not all of them latch. Check the fitting on your own machine before buying a specialty set.
From needle size down to hook timing, in the order worth checking. Each cause has a tell, so stop at the first that matches instead of replacing parts.
The dial loads a spring against two discs, and the foot lifter opens them. What each part does, and why foot position matters more than the setting itself.