Dan,
Mom used to use #50 Coats & Clark's thread when I was a kid. That was her favorite size and brand of thread.
I started sewing and messing with machines about 6 months ago I cannot find the old size #50 like she used to use. So I've been trying different brands and sizes of modern threads. Most of them are so fine, thin, and weak it's pitiful. And even those that give the # sizes on them do not equal the threads of old.
We won several batches of thread on GW mostly for the wooden spools, and many of them were the old #50 C&C thread. Some were even brand new, never been touched.
I threaded up several of my machines with the old thread and my oh my what a wonderful difference it made. Beautiful stitches like mom used to make. Then when I was working on the tensions of my Singer Mdl 66 cabinet machine it said in the manual to use #50 thread to set the tension. I used some of the old thread and you would be amazed at how much better that machine sews.
I have found that the thread makes more of a difference in how a machine sews, given the machines are in good order, than weather or not the machine is a straight or zig-zag stitch.
Joe