Well, if there's a HEF prize for best guess about an Elgin sweeper gutter broom, the winner is.......Cmark! You nailed it. We got back into it today, and found it to be almost exactly what you described. It is a shear pin, but rather than joining the two sprockets together, it just joins the gutter broom sprocket to the jackshaft bearing. All that's required is to remove the 7/16ths bolt that holds the little retaining piece, and yank the broken one out, then, as suggested, use a punch, once the holes are aligned, by feel, to push the broken end out the back, and install the new one. The best part of the story though, is this--the machine came with a bunch of filters and whatnot stashed in the cab. Looking through all that stuff, we found a dirty plastic bag that has about ten of the shear pins in it, so we got her going without even having to chase down a new one. (Ignore the filthy thumb and fingers.) Thanks, Cmark.