White vinegar (5% acidic) also works well. I used this to remove residual rust and crud in my cooling jackets that resisted two hot tankings. The vinegar is cheap (about a $1.50 per gal) and rinses off with water. If you decide to do your cooling jackets you need to put the motor on one side at a time on a motor stand. Then buy some cheap rubber expansion plugs to seal the core holes. Then just fill from the water pump port with vinegar untill full. A couple of days will do it. You'll be surprised at what comes out.