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As Mahlon has stated it is not unusual to have to bend the inner coil of the mainspring to get a tight fit on the arbor, this is quite common as both ends of the mainspring are left soft so they will bend much easier. If the inner coil has been opened slightly you will have more trouble getting the tang on the barrel arbor to catch when you try to wind it up again. use a pair of fine pliers and bend that inner coil back in tighter but make sure you keep it round as you can have even more of a problem if you bend that first coil out of shape.