The gear on top of the starter is the pinion gear. It can jam from a number of reasons. The teeth can be worn out , the spring on top of the gear could be broken or lack if lubrication. There are some grooves on the shaft the pinion gear rides on. Put a very small amount of grease, very light on these grooves. Turning the gear backward, you may have to turn the flywheel backward an inch as well, push down on the gear to free it. Run it up and down a few times to lubricate the gear.At the same time check the spring that it is intact. Look at the pinion gear itself for excessive wear.If the gear is worn out it can be replaced without having to replace the whole starter.
Hope this helps.
