I tend to over-think questions like this...
Your/this issue/reason/context is very unclear and non-specific.
What type of recip-engine?
single/dual0cylinder [conversion]?
Opposed 2-4-6-8 cylinder aircraft?
Inline-cylinder aircraft
inline or V-auto engine [conversion]
Radial [small, large]
Carbureted, injected, 'super-charged-air'
Storing the engine 'on-aircraft' or on a transportation cradle or in a sealed storage container?
ARE YOU looking for simple preservative [oil, compound, etc] for recip-engine cylinder protection for short-term or long-term or indefinite storage... thru a spark-plug hole?
ALSO...
What about the crankcase, valves, connecting-rods/pins, crank and cam shafts, carburetor-injectors-turbocharger-supercharger, gears, magnetos, etc, etc, 'below' the piston rings.... and the interface at the piston rings [your question?] and de-preservation... assuming I understand the elements of Your question...
Continental, Lycoming, P&W, etc have processes for engine preservation... which usually involves slowly rotating the engine with the starter, ignition off with a preservative... and special/important procedures for 'recommissioning' the engine after storage, as I recall.
The following book may also have valuable insight into this matter...
Sky Ranch Engine Manual: Operation, failure, repair, piston aircraft engines
I have a teaching scenario regarding corrosion preventative compounds [CPC], lubricating oils, lubricating greases and solid-film lubricants [and hydraulic oils and coolants, etc] for my young engineers, that encompasses the vast array of factors... including elements/limits of preservation... but is primarily oriented to MIL turbine aircraft.
Regards, Wil Taylor
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