Here’s an intake and exhaust valve in a cutaway R-4360 radial engine, the largest piston aircraft engine ever put into production. It had 28 cylinders thus 28 pairs of valves and 56 spark plugs per engine. High octane aviation gasoline of the era was boosted with tetra-ethyl lead, which in addition to causing brain damage would also foul the plugs badly. 4-engine planes would have to replace 224 spark plugs after each flight.

Some more valve lore: In normally aspirated engines (i.e. your car) the exhaust valve shuts completely before the intake valve opens. Supercharged engines like the R-4360 are “cross valved,” where the intake valve begins to open slightly before the exhaust valve shuts. The positive pressure of the supercharger blows a cool air/fuel mix into the cylinder and briefly out the exhaust port. This provides scavenging but also cools the exhaust valves themselves, typically the hottest part of the engine and the most prone to catastrophic failure due to overheating