A problem that can occur, is that the water (expanding ice) doesn't know an acci-"dent" from an intentional "dent" (like the expensive to create contours that run along the tank's bottom edge). The tank " contour" that's over front rocker covers of a Knuckle & Panhead will most likely get larger. The ice may also push emblem indents out from their centers. And, the alignment of the top and bottom holes for the shut off rod might get moved, left unsupported against the ice expansion.
Results:To get the dent out of that other one, fill it with water and let it freeze, it will push the dent right out and will cause no damage. The emblem mount might not work, but who knows, it is worth a try. Just wait a few months and the weather will be just right, let mother nature do the work and as an added advantage it is totally green.
I had actually heard of this years ago but I was a little apprehensive about doing it to a $500 gas tank and I must admit I was a bit concerned when I saw the bottom pushed alittle - especially since I could see that from the house. -28 here this morning.