I helped another Legacy owner troubleshoot a similar problem. The engine would turn one or two blades then stop and the contactor would start to chatter on and off. These are the classic symptoms of a weak battery, but he had just installed a new battery a few months before. I put a meter across the battery – the voltage would drop to 3V while cranking and wiring would heat up rapidly. It turned out that the starter was shorted internally.

 

This was a stock full-size Continental starter but the lesson is clear – don’t rule anything out and keep an open mind when troubleshooting!