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    This is an odd little detail in the plans. There are six solid rivets that get installed in the spar tunnel. They are the only solid rivets used other than the wing spar solid rivets. The install, three on each side, from the top.  The other side of the holes have been countersunk and the upset head is actually formed inside of the countersink. The wing spars come in at an angle and if you used regular pop rivets the stem would interfere.  I held my bucking bar on top of the rivet head and just hammered the upset head.