machinehead

At least in the video, the needle appears to be in wrong. Thread the needle from the left side toward the hand wheel not front to back. Longest groove in the needle goes to your left hand. Also, there is a sort of triangular projection on the bobbin case. The thread comes out of the bobbin case slot in the thread tension spring and goes under the triangular tab and to its left.

The original needle system called for by your machine is either 16x4 or 16x246. These measure 33.9 mm from the end of the shank to the eye, or just a little over a mm shy of 1 and 3/8". Like Wizcrafts says use #69 thread and a needle with an eye neither too big or too small. You can read about a neat trick to choose the proper size needle and thread combination in this forum. There is an online manual for your machine on the Singer Industrial Products website.

What class of needle are you using? Do you have the long groove in the needle to the left?

Over the years I have purchased many needles for my vintage industrial machines and sometimes I have bought several boxes I don't need or can't use just to get some that do fit my machines. In some cases, I just have more of a particular needle system than I need. The end result is that I have a treasure trove of needles from the common to the rare that I need to sell. Offering them here first. PM or email me with your needs and wants and I will get back to you as soon as I can sort through them for you. PayPal for payment, shipped USPS Priority, Flat Rate, or First Class whichever is best for your securely packed box. Thank You, David

Andy, Welcome to the forum. You have certainly come to the right place to learn about sewing machines. There are several very knowledgeable members who are very willing to help. As it happens, a member is currently working on a post which will address many of the questions that arise on the proper machine for a particular type of leather project. Now then, about the 331k104. It was designed as a tailor's machine to stitch shirts, blouses, and suits. It stitches more than 2000 stitches per minute. When stitching leather, you need to go slow and easy; especially when you are just starting out. Plus, speed equals friction which isn't good either. That's why a lot of stitchers have a thread lube pot on them to help the thread pull through the leather easier and help it stay cooler. Does the machine come with a clutch motor? If it does it will take off very fast when you hit the pedal, unless you are already experienced with such a set up. Leather machines often have a speed reducer or a servo motor or both to slow things down. The 331k104 doesn't have a very high lift on the presser foot and unless someone has already replaced the feed dogs, needle plate, and presser foot with a roller foot kit it won't feed the top and bottom layers of leather evenly, so they won't match up at the end of the seam. With the roller foot kit installed this machine could be useful to sew up vests and chaps. If you think you might get into that sort of project and the machine is in good shape and cheap enough, you might do well to consider it; but even then it isn't the very best machine for that purpose either.