ElMariachi

I just replaced the starter motor in my '76 280z. The original symptom was that the starter would spin fast but not engage the flywheel. The ring gear has some wear on the teeth edges but no broken teeth. The new starter does the same. I pulled the plugs and put a hand switch on the solenoid. When I trigger the solenoid the motor will turn the engine a little bit and then the starter motor will spin but not rotate the crankshaft. Any help would be greatly appreciated. Thanks.