I've compiled a table of the crack reports I've seen in this thread so far, and posted it on Google Drive at
this location.
The first column in the table is the post number in this thread. The next column is the aircraft model number following "RV-". The third column is the number of hours on the airframe. If you said something like "500+", I just entered 500. The fourth column is the number of cracks reported. Out of 41 airplanes with reported inspections, cracks were found in three (or maybe four, inspector wasn't sure). The fifth column is the location of the rivet factory heads (opposite the shop heads), if reported, FWD or AFT of the HS rear spar.
A friend who is building a -7A says his drawing shows the manufactured heads on the AFT side of the spar, i.e., on the hinge bracket side, not inside the HS. We both recall an instruction we saw somewhere that says the manufactured head should be on the side with the thinner material, but there are reasons to put it on the hinge bracket side in this instance (e.g., elevator leading edge rubbing). Also, neither of us recalls that instruction saying anything about steel vs. aluminum.
Please PM me if you spot any errors.