About the 204-420
The 204-420 is a different kind of machine from the walking foot flatbeds above it on this page. It uses a barrel shuttle instead of a rotary hook, which holds a large bobbin of 277, 346 or 415 thread and forms a lock stitch that a rotary hook cannot manage at those sizes. The 794 needle system goes up to size 230, the stitch to 12mm, and the foot lifts 24mm.
The 16 inch throat and the slow 600 RPM top speed are deliberate. This machine is for cargo slings, seat belts, saddle pads, harness, heavy canvas and layered webbing, where each stitch has to pass through half an inch of material and the operator wants to see every one of them. It is a triple feed head, so the layers do not creep.
If your thread is 138 or lighter, this is the wrong machine: a 1508NH is faster and cheaper. If your thread is 207 and up, it is often the right one.
