Joe, You're going to have tension (& stitch quality) problems with that heavy duty thread you always use. If your tension & stitch length are set correctly & you're using the proper needle, all purpose or machine quilting thread is plenty strong to make a denim quilt. And it won't cause the problems that you've been having with tension, etc. Personally, I'd be using a normal weight thread & a size 14 needle...always use the smallest needle that'll do the job. It goes thru the fabric easier & doesn't poke a larger hole than necessary...actually making weaker rather than stronger seams! I only use a 16 to hem jeans or do flat felled seams in denim where I'm sewing thru many layers. And in those applications, the heavy or top stitching thread is usually not used in the bobbin. It's pretty much used for topstitching only...that's why the jeans thread has so little yardage on the spool.