To sew with heavy thread in leather you need a LEATHER point needle with a big eye. A 16 may not be big enough for your thread. You may need an 18 or 20... An 18 or 20 is less likely to break, too. If you use a needle with a big eye you have to have a big enough hole in your needle plate for the needle AND the thread to pass through or you are right it will not make a loop for the bobbin. I think a lot of the times they are really pushing it to be sewing that stuff on the household machines they are advertising. I'd use an industrial walking foot. That said I have sewn leather on an ancient Singer 15 I had rigged for hand crank. The big hand wheel was what it took to get enough momentum to pierce the leather the HC went slow enough to control my work. Even then the stitches weren't long enough to suit me. You can not enlarge some of the needle holes for a 20 or 22 needle because they are so close to the feed dogs and you might not get it centered like it would need to be. Yes, you will need to tweak your tensions.
Even an industrial machine will only go so thick with thread. There are special machines out there for the really thick thread.
Last edited by miriam; 09-18-2015 at 05:25 PM.