with a bike with a electronic ignition would need test equipment and crank engine to know, i think. an old bike that had points you be able to see which plug would get spark. and most of the time the lead are labeled.
the locating pins are to locate rings and to stop laterail movement.
the grooves house the rings and allow the pressure in the combustion cylinder to get behind the rings to form a seal.
this is all off the top of my head and may not be technically right.
im assuming you want to see the firing order of an engine?simple way i suppose is to remove the spark plugs and get someone to continuously turn the rear while ( of bike ) and have a look-see at which piston is first to get to the top, then second and so on and so forth.
+1 Cams will tell you' its only going to fire when both valves are closed. Thats only important for EFI engines though, CDI engines are (AFAIK) all wasted spark, which means that one coil pack connects to the inside two cylinders and the other connects to the outside two.
The piston ring grooves hold the piston rings, without them you wouldn't have oil sealing or prevent the combustion process from just bypassing the piston into the sump
Piston ring posts are to locate the piston ring in 2 stroke motors so that the ring doesn't rotate around into the port area, the reason this is important is because if it does then the piston ring can open up into the port area then the next time the piston travels up (happens quite frequently) the piston ring will be snapped. In 4 strokes its a non issue because you don't have ports, so there aren't posts.
EDIT: I'm assuming you're asking about an I4 engine for the firing order, for a single its pretty simple and for a twin you just check the cams again.
the piston rings with grooves are to seal the combustion chamber and a lubricant in a way
when the piston goes up, all the oil in the cylinder from bdc to tdc is collected due to the grooves in the ring and then this oil flows through the piston(lubricating and cooling it in a way) back to the sump
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