Ford recommends you check the installed height of the gear, whenever installing a new distributor (or a new gear on an old distributor). Also, a distributor gear should be a press fit onto the shaft. If the gear's loose on the shaft, there's too much load on the pin and it will fail in short order.
These are both common problems. A friend of mine bought a brand new Mallory distributor. When he checked the installed height of the gear, he discovered it was indeed too low on the shaft. When he removed the pin to rectify the situation, the gear literally fell off the shaft.
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