My knowledge is pretty good, however I have never had much to do with starters apart from fitting them or taking them off. Two years ago my starter got slower and slower so I took it off and opened it up, I found inside a load of gunky oil, a broken brush spring and a rather well worn brush. I cleaned the whole thing out thoroughly polished the armature, fitted a new brush and spring, refitted starter and away it went like a wee birdie
Last week I went to start the tractor, it was dead
Took the starter to an old retired electrical engineer who promptly told me I had not tesioned the springs on refitting them.Almost as if magic it had worked ok for two years even though the brushes were hardly even touching the armature, took it apart again and cleaned the armature refitted the brushes and springs, this time properly tensioned,starter back on and spins once more like a little bird. Let this be a lesson to all aspiring Dexta engineers 