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Quick summary
When the ignition switch is turned the solenoid is energized, pulling the plunger back and pushing the drive forward at the  same time. The plunger hits the moving contact and the motor spins. If it doesn’t, don’t scrap or strip your starter motor yet.

Back to Basics
The starter motor engages with a gear on the flywheel of the engine, to crank the engine, in order to make it fire.  

The Solenoid
A solenoid is mounted above the starter. Some purists call this bit an actuator, but no one in the motor trade does! When the starter is switched the solenoid pulls a plunger into itself. The plunger is connected to an engaging lever that in turn is connected to the starter gear (known in the trade as ‘the drive’). So switching the starter pulls a plunger in which pushes the drive out into the ring gear. At the top of the solenoid are a set of 3 contacts. One comes from the battery, one goes to the starter motor itself and between them is a moving contact. The plunger (which I talked about in the previous paragraph) comes back and pushes the moving contact onto the others, thereby connecting the battery to the starter motor. 

The Armature
This sits in a field housing (or body – yoke). There are two types of field housing; one with permanent magnets, the other with field coils which are wound around ‘pole shoes’. These pole shoes only become magnetized when a current flows around them. Most field coil windings can be replaced - although they are often hard to remove because the pole shoe screws are normally very tight indeed. The permanent magnet version suffers more from un-stuck magnets (which can be stuck back) - but don't turn them round. Broken magnets are normally caused by being tapped with a hammer.

The Commutator
The commutator is usually found at the back of the armature. The brushes bear directly on this to transfer the electrical current to and from the armature.

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