My starter failed with a fractured magnet and a kind forum member @boxerscott sent me a spare parts one which 'had problems' which he had long since replaced. I figured that I would only need to change the housing with magnets to get a working starter motor.
It turns out that on both of these starters the grease flinger disc had become loose on the armature and the magnets had pulled the disc towards the winding and worn away the insulation causing the armature winding to arc through the disc, effectively dead-shorting the unit. Here is the armature removed from the starter and the rather battered disc, you can see both where the copper is exposed from the armature rubbing on the loose disc and the hot spots on the disc where the arc has been passing through it. To remove the armature you have to remove the brushes then gently pry the plastic end housing off the metal casing and copper posts. Pull off the grease cap and remove a small clip that holds the shaft in.
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Now if you have a starter that's been messing about when cranking then gone on to just rapid-clacking when the button is pressed, this might be the cause. A short-term fix would be to strip off the motor and remove that plate, then there is nothing to short against but equally there's no longer any barrier between the motor and the epicyclic reduction box.
My remedy, because I have the tools to do it, was to re-insulate the windings and make a new flinger. I got some winding sealer left over from another job and painted it over the exposed winding ends
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Whilst that was drying I turned a new flinger from all bar, and pressed it on to the armature stopping just short of touching. The flinger is 1mm thick and the hole in the hub 12.3mm
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Re-fitting the armature into the housing. It has 2 thin washers and a small clip under the grease cap.
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Scrape out the dry grease from the grease cap, add about 1/4 full of fresh grease and gently tap it back in place
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When you re-fit the brushes you'll have to hold the housing in place whilst re-fitting the metal end cap, or the brush springs will push it upwards. It's a simple task.
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So, with the new flinger in place (and not shorting the windings) the starter went back in and works perfectly once more. As I said above, in a pinch you might get away without the metal flinger for a while, though I suspect that it would eventually get fouled with debris from the gearbox. You could even try aralditing the disc back on with a paper shim inside to stop it shorting on the exposed copper, if you fancied it might work.
Quality post. Practical and technical.