hmmmm Man, I hope my recently found longarmer uses poly to quilt with lol

I'm not sure about machine quilting, but in hand quilting if you dont stretch the quilt well while quilting the threads will break when you use it because when you pull the quilt over you it stretches the fabric more than the thread can and breaks the quilting threads. hope this makes sense

I use cotton thread for pretty much everything. But I do use the Maxi Lock once in awhile (its poly) My 2 cents: when quilting use a longer stitch length and a little looser tension on the top works for me BUT each quilt is different and each of us is different. Oh and I never use C & C thread.

Coats & Clark thread is made with short-staple fibers. Long-staple threads, such as King Tut, are stronger because there are fewer places for them to break.

Also, I am wondering how far apart your quilting lines are. Quilted lines 8 inches apart will be under a lot more stress when someone sits on the quilt than quilted lines 2 inches apart.

The "coming undone" problem may be related to how you fastened your ends. If you did not knot and bury ends, or stitch in place for a couple of stitches, or make tiny stitches at beginning or end of your line, this would allow the thread to come loose.

Although you can use heavier weight yarns for quilting (such as 30wt), it's more common to use 40wt or even 50wt.

Cotton thread doesn't have the stretch that poly thread does, so tugging/pulling on a quilt, agitating in the washer can cause threads to break too.

I use primarily cotton thread in all of my work and I have not had anything break - and the quilts have been through the wringer.

Do you still have some of the thread so you could do a tug test? You should not be able to tear off a piece of the 30-weight, at least not easily. I suspect that your quilt stitches might have been too small and that the tension was off.

Can you requilt it? If you don't want to see the second set of stitches, simply use a good quality polyester (NOT Nylon!!!) thread.

If you have long runs of threads against the bias, there is nothing that the thread can do but break, no matter what thread you use. I've also seen seams split when there is very little quilting on the quilt, and the seams have to take all the stress.

No, the popping/breaking usually occurs in the middle of a line..Not on one of the ends ( that might not be locked down good)....And my quilted lines are spaced around 3 inches apart...
I honestly think it's a combo of the cheaper thread (and not poly)and not a looser tention... What's the typical tention setting when quilting that you guys use?..As I said, the guy set mine at 4

You need to adjust your top tension to suit the specific thread you are using. 4 on one machine could be 5 on another, so that setting doesn't mean much. What is important is that the tension be set on your machine to suit the thread you are using.

You can check tension by examining some stitches to see if you have a balanced stitch -- bobbin thread should not show on top (or only be tiny dots) and top thread should not show on bottom (or only be tiny dots). Basically you want the interlocking of the top and bottom threads to be exactly in the middle of your fabric thicknesses.

If the top tension had been set too high, your thread would have probably broken while sewing. It doesn't explain why the thread is breaking after it is in the quilt.