I use Kreinek (sp) thread Ninnie.. yep 100% cotton, and I use the hand quilting kind so it's a bit heavier also.. and yes.. sometimes I have trouble threading the needles.. Have to have a Bright light and the strongest readers I own.. I thread a whole tube (the needles come in tubes) on the spool at once. With a thinner thread it might not be such a challenge. But I sure do love those needles.