I use a propane torch, and keep it moving around the top 1/4 or 1/3 of the piston never leaving it in one spot for very long. When the part you have the torch on begins to glow dull red, I continue on around the top a little more and then dump it in the oil. Orwick pistons do have like 2% nickel. I do not know if that would be a factor. Some have already been heat treated like Madewells, and will not grow. Others like Super Cykes grow quite a bit. I have never tried any thing smaller than 60 size pistons, except for one Red Head McCoy 35 that it did not work on. They expand better by tapping the dome with a hammer to expand the top of the piston. But carefully as the sintered iron piston can crack.
I have yet to ruin a piston by heating with propane and quick cooling in oil, but that does not mean it could not happen. But, if you have a worn out engine anyway, what have you got to loose? It don't cost nothing to try. And if it does work, you will have a piston that will seem to last forever. At least that has been my experience. Your mileage may very. The Super Tech oil will clean out any carbon from running castor. Plugs stay clean forever.
I have been breaking in a couple Super Cykes that I just did, and they both have super compression where before they were OK, but not great. The one I had to do nothing but put it together, and the other one I had to lap as it was to tight. I used Turtlewax fine polishing compound for the last little bit at the top of the cyl, to leave it tight for running in. It worked extremely well. They both have good compression hot or cold. A little better cold. But start very easy either way. Usually one flip hot or cold if primed right. All my sparkers have been running on stove fuel with 25 to 30% Super Tech 2 stroke oil from Walmart. It runs very cool and does not leave a sticky mess.