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After fitting the aft ribs to the main spar they need to be fitted to the rear spar.  The first picture is after they have been fitted and are ready for updrilling.

The second picture is after the aft ribs have been fitted.  They are removed and the forward ribs are reattached.  At this point the spar holes are all final size, but the forward ribs only have two holes that have been updrilled.  After attaching the ribs just updrill the remaining rib holes.
      These two pictures just show the spacers at each rib location. The first shows what is typical at eight stations, a thick and a thin spacer. The second shows what is used at two stations, just a thick spacer between the two web pieces.