Finally started testing sewing for backpack, got thick nylon thread as recommended. I'm in the USA, so any suggestions ideally should be readily available here.
The problem is the bottom thread always lies on the bottom, after examining sample seams with a magnifier, I can see the problem easily, the thick threads when they knot are simplyy too big for the hole created by first a 16, then 18 size universal schmetz needle.
First I thought it was the fabric, but tried it on 200d oxford, and it still lies on the bottom even with tension cranked to full. This machine, a heavy kenmore from the 70s, has no problems sewing the actual stitch, the hole is just not big enough from what I can see for the knot to pull up through into the center of the materials. I've adjusted the bottom tension on bobbin holder too, but when I examine the holes, it's fairly clear to me that dense nylons simply will not let the big knot through when it's bigger than the sewing machine needle hole by a significant amount.
I did some reading here last night, found one person suggest that maybe something like a Singer 18 jeans needle would be better. I can't find bigger needles so I don't think that's an option, but when I examine some backpacks, a Lowe Summit pack, for example, I can see that they use really big needles, 22, 23, and even thicker thread than I'm using from the looks of it.
I tried now a few different heavy nylon types, Coats & Clark Extra Strong Upholstery, Super-Tuff heavy duty nylon sewing thread. I also got the Gutterman heavy polyester, but that looks to be even thicker than the nylon upholstery threads.
Someone here recommended c and c dual duty plus thread, which I think one fabric store locally has, they had the upholstery stuff. I can also check out Nareens in Berkeley today.
Is this a problem do you think of the needle type, brand, thread, or combination thereof, and how can I solve it without buying endless amounts of heavy nylon threads and needle assortments?
The machine itself is pretty powerful, old school heavy duty, and I mean heavy, sews other fabrics fine with standard gutterman threads and needle sizes.
Sometimes in online info sources they talk about various thread types, but those pieces of information do me no good since the thread I can actually buy doesn't give me that information, like tex number etc, so the specific brand and type is what I need if I need a certain thread.
any thoughts appreciated.

