Got my new heads but...

Gas it too dangerous, and it is too volatile. It will evaporate too fast skewing the results.The gas will creep through the tiniest crack, that will make no-never mind to a running engine. You will be fighting a non-issue for days.
Kerosene or diesel molecules IMO are too big, and they leave behind an oily residue, plus they stink. But they may/will work overnight. The oil in the kero creeps sideways pretty good, but not so much down. And then you still have to clean it up when you're done.
Common parts-washer solvent works just right;results in 20 minutes.I have not found a better commonly available test-fluid.
Paint thinner, the more purified form of washer-solvent will creep almost like gas, so I stay away from that too.
Water-molecules are way too big and they stick together like glue. Just try pouring water through a tiny straw...
I don't actually fill the chambers.I only cover the valves, or the perimeter of the valves. I watch for the creep, in the ports.