Cleaning water jackets 225

I doubt a rebuilder would not first hot-tank the block. Anyway, for in-car cleanup I bought citric acid on ebay. Mix ~1 lb per gal and drive for a day or 2. But first, totally flush out all coolant to pure water and again aftewards. I think the same stuff was in the better Prestone flush which you can't seem to buy anymore. Be careful using vinegar since one guy reported it attacked the cast iron when he left a head soaking in it for a week. Also, pull the block drain for the final flush (passenger side, 3/8" NPT plug) and dig any rust gunk so it free flows.

I have done this before switching to Evans Waterless Coolant, but you must totally dry out the innards, so need time, and best to do during a heater core and/or radiator swap.