Denso starter on 87 fourtrax honda. Took it off n installed new brushes. after that it went wrong direction so would freewheel in crankcase and never engage. The cure: I thought it could only reassemble one way because all the pieces are 'keyed' or 'tabbed' for alignment, but I had the case - that is the stator - backwards -turned it around 180, hooked it straight to battery to test and ha-ha - it actually spins the correct way once again.
Never disassemble anything without a system of marking parts for the order they go in. Anyone knows that.

 

Just went through this today starter stopped working so I took it apart cleaned it put it back together exactly the way it was and it spun backwards and would not engage the motor I turned the casing around on the starter motor then it spun the correct way but after sometime it spun backwards again repeat process fix the issue but once again it's going backwards replace the starter fix the entire issue 30 bucks for a new starter. I'm assuming it has something to do with the brushes mine were very warm and so was the contact points extremely war down had a groove cut cut into the bushes and contact point

 

Polarity has been reversed. If you changed the brushes you may have assembled the starter incorrectly. Check the rotation in a bench test. If it’s going the wrong way chances are you re-assembled reversing the polarity. Change 180 degrees and bench test again. If it rotates properly you have fixed the problem. Happens often when replacing brushes as the casing is not marked to show how to re-assemble.

 

I was dealing with this issue, but on a polaris. Either way the NEW starter motor and bendix was spinning I the wrong direction. New solenoid also. I tried many different things to fix it with no luck. My fix was, I took the old starter cleaned it up and used the new cap (the part with the two square magnet's). I re assembled it and it turns on the right direction now. Works perfectly fine. My guess is it was manufacturered incorrectly from the factory. Hope this helps.