Something I used to see in the old days was folks using a 'air hold' fitting that you could put in a spark plug hole.
This thing was used to presurize a cylinder to hold the valves up while you changed the valve stem seals without pulling the head off.
It screwed down into the spark plug hole, then you plugged a air hose onto it.
It also works backwards too.
Put it in a spark plug hole, connect a air hose to it, start the engine, and you have a real handy air compressor.
"Gone are the days of wooden ships, and Iron men.
I doubt if we shall ever see thier likes again".
Author: unknown member of the USCG.