There are general rules that apply to the threading of all sewing machine bobbins.
The bobbin is filled with a thread identical to that used for the upper threading.
It must fill in level layers and must not “spill” over the sides of the bobbin.
Always empty the bobbin of other thread before filling so that the thread can be started on the bobbin by passing the thread end through the eyelet on its side.
This method provides even handling of the bobbin thread to the very end, and is important because the quality and regularity of the stitch is influenced by the free flow of the thread from the bobbin.
Careful storage of the supply of bobbins prevents them from being bent out of shape. A bobbin with bent sides will unreel with alternate heavy and light tension causing an irregular stitch.