Ken; One style of after market chopper is known as the Redekop chopper. I use one myself on a 1660 and am very satisfied with it. It is an external mount, attached to the rear end of the combine. There is also a slow-down kit for corn if needed. Here in Iowa we have a distributor, GandD combine, that handles new and used choppers for IH and Deere. The number is 1.888.387.2627 ask for Denny.