I've used threads of all kinds, different weights, etc. without any problems except for rayon. Rayon seems more suitable for machine embrodery than FMQ. It kept breaking for me so I don't use it anymore. I have had some rather biased quilting teachers tell me not to use Coats and Clarks. I don't listen to them. Coats and Clark have been around for years, and that is good enough for me, smile.