Won't start

[nickt5174]

Hey guys having this issue and hope someone can help. So my x5 has been sitting for awhile and the battery died. So I bought a new battery since it was old anyway and hooked it up, tried to start it but the starter wouldn't engage. I hit the key and all I hear is the starter spinning but the motor doesn't spin.

OK so now I need a new starter so I replaced the starter, bought an advanced auto unit for $130. I have used these same starters before in two other BMW's that I have owned and they have worked no issues.

So I replaced the starter myself got the car back together, hooked the battery back up, hit the key and the same thing happened. I can hear the starter spinning but the engine does not turn. WTF

So before I rip the whole thing apart again because maybe I got a bad starter,
Is there ANYTHING that I may have missed in regards to replacing the starter?

It's the same problem I had before, I can hear the stater spinning but it wasn't engaging the motor.

Thanks for your help