This is a picture of the rear crankshaft on a N55,,,,the reason I’m point it out is there is actual a ring that goes around the rear of the crankshaft,,,that ring is magnet and what it does  is help tell the DME where top died center is,,,,,you can see the black sensor on top which gets its reading off the magnet ring. Here the ring removed from the rear crankshaft,,,,now this gets sandwiched between the rear crankshaft flange and the flywheel. Now this is the important part this grey edge is where the magnet is,,,,,do not damage this edge,,,do not clean it with carb or brake cleaner because it will damage it,,,,do not forget to put it back on the engine because it will not start without it,,,,I have this happen many times when replacing a new engine.,,,,this little thing can ruin your day if you damage it of forget to put it back on the motor.