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Can you drill the bracket also alternator hole and use a larger size socket/cap bolt?
Even if the cap head was slightly too large, could you machine the outside down for clearance?
Is this bolt the one the alt' pivots on or the adjustment/tightening bolt?
If adjustment bolt, is there enough meat to make the alt' hole larger or oblong so bolt sits off ctr. w/ head further way from blades when tightened down?
~ Pete
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