The smallest needle size that will accommodate #138 thread is #22/140. A size #20 needle will not sew #138 thread! If you are using a #160 needle, you have to enlarge the hole on the feed dog and move the hook as to left and right. I did this on my Pfaff335 and the machine sewed fine on materials. However, I do not sew veg tan leather on this machine!
glenn