6800 shifter diagram:
https://si.shimano.com/pdfs/ev/EV-ST-6800-3603.pdf
8000 shifter diagram:
https://si.shimano.com/pdfs/ev/EV-ST-R8000-4244.pdf
I suspect you're talking about part
#9 in the diagrams. You can do a search with the "Shimano Code Number" to find sellers and yes, it's needed.
Last edited by Crankycrank; 03-27-20 at 09:22 AM.