I was just looking at bruces dyno page and saw the rusty old engine block on the dyno. A really easy way to remove all the orange rust and bring cast iron back to its original grey is to get a tub or tank big enough to fit whatever the past is you want to clean and fill it with molasses (is that how its spelt?) and water. you can use up to 50/50 mix with water but the stronger the better.
Your parts need to be free of oil or it wont work. After a few days soaking, hose the parts off and they will come up perfect, if not, put them back in for a while. A place I used to work had a tank and we put old vintage single cylinder engines that were frozen together with rust and after a few days you could strip them down without a worry.
Only problem is it doesnt smell the best after a while so dont sit the tank outside your kitchen window!