Degree of Crankshaft Rotation:

All events which occur in an engine are associated to the location of the piston. Since the piston is linked to the crankshaft, any location of the piston corresponds straightly to a specific number of degrees of crankshaft rotation.

Location of the crank could then be beginning as XX degrees before or XX degrees after top or bottom dead center.

Firing Order:

Firing order refers to the order in that each of the cylinders in a multicylinder engine fires (power stroke). For instance, a four cylinder engine's firing order could be 1-4-3-2. This means in which the number 1 cylinder fires, then the number 4 cylinder fires, then the number 3 cylinder fires, and many more. Engines are designed so in which the power strokes are as uniform as probable, which is, as the crankshaft rotates a contain number of degrees; one of the cylinders will go by a power stroke. This decrease vibration and permits the power produced through the engine to be applied to the load in a smoother fashion than if they were all to fire at once or within odd multiples.