Cylinder heads are essentially a cap for your engine block. Your main block has holes machined into it to house your pistons and contain the explosive forces that power your vehicle. These cylindrical hollows are called cylinders. Cylinder heads have the primary function of sealing the cylinders so combustion remains controlled. They're also often the part in which spark plugs, valves and injectors are mounted. This makes them an integral component of the two most important engine functions: combustion and ignition. Cylinder heads are made of stern stuff, but they might fail in the case of improper resealing or engine overheating. Even on the off chance your engine starts with a faulty cylinder head, don’t attempt to drive: get it fixed immediately.