Before I bought that thread, I did a bit of reading and found several places that told me that the heaviest thread most non-industrial sewing machines can handle is the Tex 70 (a standard where I believe a Tex 70 means that 1000 meters of that thread weighs 70 g; the higher the Tex number, the heavier the thread). It did take cranking my upper tension to the max to make it work well and consistently. I thought it was too tight and would never sew without puckering, but it looks good and does not seem to struggle. When I had the tension set where I do with quilting thread, I had large loops such as the ones you describe--I think the thread needs to be very taut in order to not spring out into the wrong shape to catch the bobbin.

ETA: both Threadart.com and Wawak.com have Tex 70 nylon upholstery thread. I got mine from Wawak.

a 65 weight should work fine, too - give yourself some room.

I had to take a day off and go pick up a Wilcox and Gibbs sewing machine, it is so cute. I will get back to the 237 today.

Thank you for the help. I will try some of that 65, 70 thread probably order it today too. Also I think I will do the upper tightening with the heavy thread I was using and see if that helps it at all, I didn't tighten it down very much from the quilting spot. Didn't think it would work, then just forgot to do it the next time around because it wasn't working anyway, just slipped my mind. With all of this help I should find something to work, I am so thankful for you all. Will post a note if I am successful, well may post one if not too.lol

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

I got my thread order with the thread that MaryT told me to get today and sat down at my 237. I knew it was sewing fine with quilt thread, then put one of the ones I had tried before with a tightened tension and that didn't work. So I put in the new thread the 70 tex I just got, and it sews like a champ. I do have an 18 needle in there, universal because that is all I have that big, and I tightened up the tension on the top much more than I had and it is sewing very nicely now. Well I am excited about that, took about 2 minutes to make my first sand bag, with no problems what so ever, so this will be a fast and easy project and can get it done with no worries now as long as the 237 keeps going. It has no problems sewing over thickness, which this isn't very thick anyway, but I do think it was the other thread. It looked the same size to my eye, but apparently something is different I can't see, or the new stuff isn't as stiff and goes around the bobbin much better. I am happy with the little 237 now that it is going. I wouldn't call this a heavy duty job for a home sewing machine, it isn't having a problem what so ever, apparently it was me! Thanks all for your help on this, I am so relieved.

Just wanted to wrap this up. I got all of my bags done up in no time at all with no problems at all in under an hour and thank you all so much for your help!:thumbup: