one cause could be hydro-lock. i.e. a cylinder full of fuel, which would be caused by a plug not firing repeatedly (filling it with unburned liquid fuel). this would be a bad coil, plug, plug wire, distributor cap or rotor, ect...
another cause could be pre-ignition. a cylinder firing when it is in the wrong part of its stroke, thereby causing the explosions "up-force" to fight either the valves attempt to open, or forcing a not fully seated valve closed. this would be caused by the timing being off its mark, though your damage would be more wide spread as it would effect multiple cylinders unless this is a single cylinder engine.
also, it could be caused by a stuck lifter, though that would ultamately lead to one of the 2 above causes.
there could be other causes, but that's the most likely 3 (short of a bad cam, broken timing belt/chain, ect...).
Do you really know how an engine works? i.e. a cylinder full of fuel, which would be caused by a plug not firing repeatedly (filling it with unburned liquid fuel). this would be a bad coil, plug, plug wire, distributor cap or rotor, ect...
THIS RATES AS THE DUMBEST ANSWER EVER POSTED. WHEN A PLUG FAILS TO FIRE THE UNBURNED VAPOR IS FORCED OUT BY THE EXHAUST STROKE THERE CAN BE NO ACCUMULATION IN THE CYLINDER NO MATTER HOW MANY TIMES A PLUG MISFIRES. IF A VALVE IS NOT FULLY CLOSED THERE WILL BE NO PRESSURE ON THE PUSH ROD. PUSH RODS ARE BENT WHEN A VALVE STRIKES A PISTON RE; BROKEN TIMING BELT OR CHAIN OR JUMPED TIMING THERE WILL NEVER BE ENOUGH PRESSURE TO BEND A PUSH ROD IF A PISTON IS NOT AT THE TOP WHERE THE VALVE WILL HIT THE PISTON. Hydro- lock? not possible. The exhaust valve opens when the piston is at the bottom. The intake valve opens at the top. hydro-lock prevents the piston from reaching the top