I do not know what was used but it was toxic to the aluminium. If not already covered this would make a good sticky post. Maybe it will keep others from screwing up pistons. Engine did get water at one time. Pictured is the diesel I put in the cylinders.

Someone mentioned Muriatic Acid. Muriatic Acid will corrode iron pipes. Knowing that I personally would not risk ruining the heads and valves.