USE TWO WRENCHES! Yes, I am yelling that.

Flare nut wrenches are nice to have, but far from cheap. I tried to make one by cutting a section out of the "box" of a surplus box end wrench. Waste of time, the box just opened up. Flare nut wrenches are especially forged.

I have used open-end and Crescent wrenches for this. But - the open end must be a correct fit, meaning that if the nut is SAE, use an SAE wrench - if metric, use a metric wrench. For example: 25mm is not the same as 1 inch, though close. Same for 17mm and 3/4 inch.

The Crescent (adjustable) wrench must be screwed on tight. No slack.

Put them on the two fittings oriented such that squeezing them together will do the job, both when loosening and when tightening. Handles at a 30 degree angle.

Why all that? Otherwise the fitting on the tube will tend to twist the tube, and/or the fitting in the radiator will be twisted out of line.

Good time to replace hoses, too.

Good Luck,

Bob

:>)