The offset is important. Best is to replace both. The distance mentioned in the manuals when replacing is not the actual distance after fitting. When tightening the hub the hub touches the bearing and the then the halfshaft is pulled through the bearing when tightening the nut. This way the hub lies alway's against the bearing face. When your bearings are fitted the wrong way then there can be a certain distance between the hub and bearing face. Then the hub/brakedrum is further away from the backplate causing brake trouble and a wrong bearing load.
Jeroen
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