"Fat Thread" being an interpretation, I have only used this YLI 40/3 (Tex40) variegated yarn. It only says "YLI Machine Quilting" "Earth (the color) 13V"
Very nice. I also have put some Sulky 12 wt through the machine. To my fingers, just a hair thicker than the YLI thread. It comes in a ton of different variegated colors also.
Hope this helps.
tim in san jose