I haven’t ever went over Tex70 (Coats S964) thread in any of my household machines, but for what it’s worth, when I set my Singer 95-40 (industrial, but for fabrics) machine up to use Tex80 (Jean/Top Stitch) thread for hemming denim jeans, I had to replace the bee-hive spring in the upper tension assembly with a heavier spring to get the knot to pull up into the fabric where I wanted it....
CD in Oklahoma