For anyone who needs help with the Singer 400w21 Industrial, and I realize the OP posted a LONG time ago, I am going to try and create a YouTube video about this machine since so little exists about it. Like the OP, I couldn't figure anything out from the horribly downloaded and seemingly only version of the manual online. Threading like any other industrial machine doesn't cut it for the 400w21. This one has all of the threading guides, tension, etc. on the side, and was made in the early to mid 1900's. It definitely has some idiosyncrasies that other industrials, especially newer models, do not have.