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These two pictures show an unexpected tight fit.  This bolt head just happened to be indexed as it is here.  I don’t think the plans called out any special clearance issue.  I can just slip a piece of paper between it and the web.  If the bolt were indexed any other way it would have had to be turned to this position.

Also note the round head rivets clear as expected. You can just see the slight flattening of the round head rivet from the back riveting procedure. Sonex and EAA sources say this is just fine. The gun and bolt flatten the head  the same.

      As it turns out, the bolts the plans call out as needing to be indexed have plenty of clearance when indexed correctly.  They might even have fit indexed the wrong way!