I just recently got my Singer 115 rigged up with a 9-spoke hand wheel on it to use in one of my home treadles, but I haven’t actually used it yet except to test-sew it after cleaning and adjusting. It’s just as ready to be outfitted with a hand crank and used that way too, but I have it set for using Tex30 threads.
Before I got my Singer 31-15 treadle up and running, I used my Singer 66-3 (66-1 back-clamp with a hand crank on it) with Coats & Clark Upholstery Thread in it for canvas and thin vinyl. I’m guessing the thread size would be about Tex70, so a little larger than size V-69 but smaller than V-92. I never have used V-92 in my 66, and I’ve never used size 69 at all. It sewed the C&C alright, but I had problems with keeping the feed and stitch length consistent for a nice stitch. The feed dog would slip unless I made sure that the fabric had absolutely no restriction or pull on it, which is hard to do with fabric hanging off of both sides of the work area on long seams and hems. A roller foot is a little more forgiving, and a combination feed walking foot can pull some weight, but a feed dog alone struggles.
With that problem beside the point, I would think that the 115 would be a better machine to use instead of my 66 because of the rotary bobbin, but I haven’t actually tried it. I use V-92 in my 31-15 with a roller foot on it, and the bobbin cases should be comparable for thread size that they each will handle. But, I have no experience using V-92 in a 115. I’ll be interested in finding out how it goes.
CD in Oklahoma