I have a singer 31-15 and am just getting started with leather work. I can't find what size needle I need for my machine. I ordered dpx16 but they seem too big. I had some needles that came with the machine and they say 16x87 but I don't know how to find them. I found somewhere that dpx16 was compatible, but they don't fit my machine. I would like to run size 138 thread on top which takes a size 20-22 hole in the needle for the thread. Thank you for your help.