[Makerbot Replicator 2x] Where did this part fall from?

[jgee]

tldr: I need your guys' help identifying what part this is and where it goes.

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Hi everyone. Earlier today I noticed that my bot has started to print inconsistent circles. They're coming out oval-like. As recommended, I tightened the x-axis belt. After loosening the 4 screws attached to the motor, this part fell out. It looks like a pin/dowel/barrel of some sort. Can't seem to find out what it is and where it goes. Out of curiosity, I ran a test print and while I'm still having inconsistent circles, the printer is able to print. Anyone know what it is and where it goes?

While I'm here, anything else that might address my circle-printing issue? I've already checked the allignment of gantry and tightened the x and y-axis pulley and belt but this hasn't helped.