I am having the same issue with a TrimRite 543. There is supposed to be a shoulder washer that fits on the 3/8 inch shaft with the shoulder providing the 1/2 inch to support the blade. On my current shoulder washer, over half of the shoulder has been worn away and so will no longer properly hold the blade in place. I checked at a lawn equipment repair shop looking for one that would work but it seems there is no way to get a new shoulder washer of the correct dimensions anymore. It seems to be a part specific to this family of Trim Rite edgers. I've searched the internet for a sholder washer or bushing of the right dimensions with no luck. I'm curious if anyone else has been able to find a substitute part.