You can always use electrolysis. place a scrap piece of metal in a plastic tub. connect a positive cable from a non-automatic battery charger to the scrap metal. Place the negative cable on the manifold. put it in water that has had baking soda or lye added to the water. keep the two parts seperated, turn on the charger and watch the magic work. Let it go overnight and pull out the part. Wash it with a stiff bristled brush, rinse, and dry. The process will remove rust from all the pores, inside and out. If using lye, it will also remove paint. Works on all types of metals. Used for all the parts of the last tractor that I restored. Saves lots of elbow grease and money.
