The Singer 401A is considered by many, including the Singer Company at the time, as "the finest zigzag sewing machine made". The company had good reason to be proud of their 401 though. It was built with precision and had scads of built-in stitches, plus it had the option of using Singer top hat cams. To top things off, it was 100% gear drive.
I recently acquired a second 401A machine and decided to prep it for sale. After popping the top cover off, I noticed the cam stack was quite discolored with old oil remnants and decided to venture into the unknown realm of cam stack removal. It was actually quite easy. The top screw that holds the top hat spring clip is all that holds the cam stack in place, so once that screw is removed, it is just a matter of getting the cam follower fingers out of the way and the stack lifts free of the machine.
Three small screws on top of the stack hold all the pieces together, but after 50 years of being clamped together, it took some gentle persuasion to get all the cams apart.
The cams are numbered 1-7 except for number 5, which had no markings whatsoever on it. The cams have three holes each (small, medium and big) around the large center hole, so it is pretty much impossible to get the stack back together the wrong way, as long as cam 7 is on top and all holes line up according to size.
Here is how I started reassembly.
I lightly oiled everything also so the cams wouldn't rust. The next photo shows the cam stack back together in one piece.
Next thing to do is to pop the cam stack back in place and re-time the stack to the needle. There is a timing mark on cam 7 that you can see below. It is lined up with one of the cam followers. This timing mark needs to be lined up with the follower when the needle is at its lowest point of travel. Then just tighten the three small screws on top of the stack and everything should be set to go. No, I didn't bugger up the small screw heads. A previous owner or technician did that.
As many owners of 401s (and 500s) know, the two posts that the cam followers slide on are notorious for getting gummed up and refusing to work properly. It is just a matter of getting them cleaned up and oiled, at which time the stitch selector mechanisms should work again. The two posts can be seen in the next photo, one on each side of the cam stack post, and having the notches in the sides. You can also see the grime in various places that hadn't been cleaned up yet.
The 401A is really quite the mechanical marvel. It is fairly easy to work on but at the same time is very complex in its combination of stitch patterns. I highly recommend either the 401 or the 500, or both. You can never have too many sewing machines. That's my motto and I'm sticking to it.
"Practice" asked the question about a spring they found in their machine. I took the cover off my 401 and took a couple photos showing where the spring attaches.
In the first photo below, the spring is in the upper right. It attaches to a small "hook" about 3/4-inch to the right of the red dot in the center of the photo, and just to the right of the brass-colored screw.
In this next photo, the other end of the spring can be seen attached to a rod. The spring can be seen about a third of the way down from the top of the photo.