Best way to clean calcium build up out of a radiator??

Michaeluser said
Plain white vinegar is good but can take a while. It won't harm anything and only smells funny. I would try a radiator flush as well. 40 years is a long time you might have to get the radiator cleaned at a shop that can mechanically "rod" the radiator out.

Good luck.

X2 Since the radiator is out of the car and you can let it soak, the vinegar will do its job without harming the copper. You may have to drain and refill with vinegar once or twice depending on the amount of calcium carbonate build-up.