Why did I spin a main bearing?

Im thinking its not a main bearing. So it died when you went to "grab 3rd", right? Is it a manual? Is it locked up or was it hard to turn over?

Did you install new cam bearings?

yeah its a 4 speed, when i got it home i checked everything with the cam, valve springs, lifters and oil pump i primed it and everything looked good. so i made the mistake of trying to start it again. oddly enough the starter cranked it fine but its a high torque mini starter, only thing that clears my headers. when it fired it took about half throttle to hold it at 400-500 rpm so i shut her down right away.