One item not mentioned is the incompatibility between Suntour Ultra freewheels and standard spaced freewheels (including Suntour). The Ultra 6 was designed to use an Ultra chain and fit on a hub and bike with standard 5-speed 120mm spacing. (Standard 6 spacing was 126 mm.) There was also an Ultra 7 that would fit on standard 6 speed spacing.

There's a big problem, if you have an Ultra 6 on a 5 speed hub and try to replace it with 6 speed freewheel with standard spacing.