When using wooly nylon in the looper(s) of your serger and/or coverstitch machine, be VERY careful to sew a short test before actually sewing on your project.
That way, if the looper thread has snapped, you don't punch a hole in the T-shirt you might working on.
And, let me tell you, re-threading wooly nylon is a B___H!
I always "pull-through" when changing colors etc, if at all possible, but if the looper thread(s) has broken it means several frustrating minutes of digging out the instruction manual, waxing the end of the wooly nylon and trying... and trying.... AND TRYING to get the thread through the stupid-almost-impossible-to-reach hole in the looper arm.
:-(