that gap in the positioner "Finger" is the reason most home machine are limited in thread weights. Use a heavy ( or fuzzy bulky) thread and that gap ( or lack of) creates extra bottom tension I tried for many hours to find a non standard happy medium with my 306 to get it to use upholstery weight thread (#69)... to no avail, in the end i got a true pin type feeler gauge and set it to manual specs, and it does work very nicely with the threads it was designed for. Fixed some of the Skipping problems I was having with wide Zig Zag stitching. I was lucky to find the Singer service manual... and the gap was spelled out for me ( in thousands of an inch)