This special flat washer and flange locknut wear against each other as the closing arm moves up/down in the field. They wear fairly quickly, since they’re only grade 5 from Deere (Exapta’s are both grade 8). Eventually they wear enough that there’s additional side-to-side play in this pivot, which allows the spring for the closing arm to pop out of its setting more easily.  The wear and resulting slop also doesn’t help the tracking of the wheel alongside the furrow, and probably drags more contaminants thru the seals as it moves side-to-side. This special flat washer (has a unique ID/OD combination) and special (non-serrated, smooth underside of flange) flange locknut should be replaced every time you’re working on this pivot point’s bushings, or whenever excess sideplay develops.
Exapta now sells the nut + washer (both gr 8) for $4.58, about the same price as Deere’s but theirs are only gr 5.

 

On another note, it has come to our attention that when working on the main pins (that attach the rockshaft to the opener), the clamping bolt must be adequately torqued or the pin may spin in the cast arbor/hanger and wear it out. Be sure to get after them with an air wrench when finished. Some people replace the bolts with fine-threaded ones to get more clamping force.