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You probably know this, but you have five similar bolts.  The four outside head bolts and this one.  The four are longer but the same threads.  So, you can test the bolt in four other holes and you can test that hole with four other bolts.  If you have a nylon or brass round brush a little bigger than the hole, you can put a little grease on it, shove it all the way into the hole and then "unscrew" it out of the hole.  Usually, if there's grit in a hole, that will get it out.