I lost the hammer spring rod pivot while I was changing my hammer spring on a 640 ND. I think it was metal, but on many of my newer J Frames, it looks like plastic are these interchangeable? If I canabalize one from one of my newer J Frames will it fit? Will it change the trigger pull? Thanks in advance

They are interchangeable. It has no effect on trigger pull whatsoever.

Call S&W Customer Service in the morning, they will likely comp you one. 800-331-0852