I have a set of SSI heat exchangers in very good condition. There is one rusted bolt that I have not attempted to remove but should not be too difficult to deal with. Asking $750 plus shipping, pickup just outside of Nashville, TN is welcome.

Bump

How and where is this rusted bolt and why do the pipes look like mild steel?

I assume heat cycling caused the rusted bolt, fairly common on these from what I understand. The bolt is up where the heat collector elbow attaches. As for the color of the pipes I would say heat cycling as well. A once over with a wire wheel would clean these right up I would think.