That is because the three protrusions from the gear case need to fit into the holes previously used by the bolts that held the impeller bearing carrier.
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Some have reported being able to remove the bolts for the impeller bearing carrier without unbolting the auger. This was done by loosening the set screws on the impeller shear bolt flange and sliding the impeller forward enough to let the carriage bolts drop out.
There must be some variation in the dimensions between the two mounting surfaces on the blower housing as someone else mentioned having to beat the gear case into place with a block of wood. I suspect this is either due to manufacturing variations or the bottom plate getting bent from use.
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