Dehydrator plug

(Reciprocating Engine Preservative Device)
A transparent plastic tube filled with a desiccant agent, such as silica gel, and an indicator, such as cobalt chloride. One end of the tube is sealed, and the other end is fitted with a perforated plug and threaded so that it can be screwed into a spark plug hole. When the air inside the cylinder is dry, the indicator dye is blue, but if moisture gets into the cylinder, it is absorbed by the silica gel, and the indicator changes its color to pink.