I just received my xcarve yesterday and had a similar issue. The ID of the bearing measures 0.310" and the OD of the ACME rod measures 0.312". I followed the advice in this thread and mounted the smaller diameter end of the rod into my drill and pressed a file to the ACME rod to remove the 0.002" excess diameter. The rod inserted into the bearing with a small bit of force after that and seems to be doing fine.