that is a difficult answer cause you need to know where and how the thread break,that means iff it break somewhere to the top section it could be take up lever or take up spring cutting thread,at needle it could mean to thick cotton for size needle,and if it breaks below needle plate it could be timing,damaged hook,damaged or wrongly set thread guide,if it fray below needle plate the most likely cause is incorrect timing.also old thread can give you endless problems,try new thread before assume bigger problems. hope info help a little
johan, May 2014
