[Joe's Garage]

When there was a lot of rust in the block and a fresh radiator, we installed an old radiator - even a leaker - and then run the chemical cleaners thru the whole system to get the rust loose.

Since you normally need to run the engine to activate the cleaner, there isn't really any other way without removing the engine and having the bare block cleaned at a machine shop. Lots of work to do that.

Usually we did it once, then flushed and did it again with the cleaner. Twice as much cleaner and twice as long to get things twice as clean.

After the two cleaning 'cycles', we flushed really good a couple times with clear water and put the new radiator back in and fill with coolant and distilled water.

YMMV. Good luck.