I recently had the right front stem on my Jenn Air cook top replaced; only the wrong part was installed. The new stem is round front and back and flat on the sides. The original stem was round in the front and flat in the back… while in an off position. Needless to say I can’t use the knob which came with the unit, so I have not been able to use the burner. I have research the issue on my own and have found what I think is the correct stem, #8 stem/part #AP4087111, but I am not sure. Hopefully someone can help.
The part you called “stem” is actually a surface unit switch. The manufacturer used two different kind of those switches for this model. The difference is the configuration of the shaft. It can be a double flat shaped shaft or a D shaped shaft.
According to your description you need the switch with a D shaped shaft. The AP4087111
you posted is the correct part number.
Here are the breakdown diagrams and Replacement parts for JENN-AIR C221 SURFACE UNIT- DDRAFT ELE | AppliancePartsPros.com
Gene.