40 wt is way to thick for piecing and rayon thread is mostly used for machine embroidery. Aurifil is 50 wt 2 ply so it's thinner the 50 wt 3 ply which most 50 wt thread is. You have to know the ply not only the weight. Then there is needle size for thread size. One size needle does not fit all threads. Needles not only have size but type. Superior threads has thread and needle education on their website. I like Masterpiece or Aurifil for piecing. I buy pre wound bobbins from Superior, cotton and poly.