A crankshaft sensor is used to measure the position and speed of the crankshaft. This assists in fuel distribution and ignition system. Many rough starting or running problems can be traced back to a faulty crankshaft sensor.
The bearings should always be inspected when removing the shaft and always need to be replaced when the crankshaft is ground. Make sure the bearings match the crankshafts grind and use the proper assembly lube when installing.