Tammi, I'm not sure about the thickness. Call me an idiot but I generally just grab a spool of thread and am happy if the color matches! I've only recently started noticing that some are cotton and some are polyester threads but have never really looked at the thickness. Some of the older spools I have don't even have the label stuck on anymore.
I've gotton so many spools of thread when picking up machines that I now try and sort what seems thicker to one area. Most of them all feel the same to me! If I get a lot of breakage with a thread I double check the bobbin, the needle and the guides for rough spots and if none are found the spool is considered too old and dumped in the trash.
Both the bobbin and the top threads on this machine read cotton/poly blends but I can't find where it states a thickness. ?? Should it say on the spool?