Mike came by on October 25, 2012 and inspected my work. He was complimentary of my project and asked a lot of good questions about my tools and my paint priming process. During the inspection of the mid-fuselage, Mike noticed that the shop end of the top row of AN426AD4-8 and AN470AD4-8 rivets on the aft bulkhead, F-1204D, did not appear to protrude far enough through the F-1204 center section channel. I had not noticed this until now. A quick check with the rivet gauge suggested he was correct.
The picture below shows two rows of rivets with the shop end of the squeezed solid rivets exposed. Mike referred to the bottom row of rivets as "overdriven". The diameter of the shop head is within spec of the go-no-go rivet gauge for a number 4 rivet. It is the thickness of the driven head that is lacking. Note in the following 2 pictures that the fuselage is currently inverted as I am preparing to attach the bottom skin.
The next picture is of the manufactured heads of the same AN426 and AN470 solid rivets.
The head-driven thickness of the rivets is about .040 inches as measured in several places along the line of rivets with a micrometer.
The morning after Mike's inspection, I called Vans to get further information. During the course of that conversation, the tech support rep never did address the possibility that the designated rivets might have been too short. Instead, he acknowledged that drilling out those rivets would be a challenge, especially now, since the baggage ribs and floor skins were already in place, which significantly limits access with a drill. His suggestion was to use a right angle air drill with a number 40 bit, followed by a number 30 bit if I elected to drill out the rivets. He went on to cite a table on page 104 of the sixth edition of the Standard Aircraft Handbook which calls for a minimum upset height of 3/64 inches (.047) for a 1/8 inch solid rivet. As noted above, the rivets are about .007 inches short by this standard.
As I further researched the issue on www.Vansairforce.com internet portal, I found a reference to the Mil-R-47196A, published September 6, 1974, which is the military's specification bible for all things structural. In Table III, under the heading of Rivets, Buck Type, Preparation for and Installation Of, Standard Flat Driven, Head Dimensions, the 1/8 inch rivet lists a Driven Head Thickness Minimum of 0.050 inches.
Finally, a search of the archives on Vansairforce yielded 2 relevant threads published by other RV-12 builders who had the same result in their F-1204D aft bulkhead rivets. One thread in particular referenced a response from Ken Krueger, former Chief Engineer of Vans Aircraft with substantial knowledge of the RV-12 aircraft. In that response Ken simply cited the Note in section 5D, entitled, "Riveting" of the builder's manual. The note reads as follows:
"Note: There are times when the correct rivet length is not available. Depending on the application a shorter rivet can be used or a longer rivet cut to the proper length. Using a longer rivet, as is, can result in the shank being bent over like a nail. We have chosen to use a rivet that may seem too short in some places, but will do the job adequately."
It was also noted in this thread that the parts in question (F-1204D riveted to F-1204) are experiencing shear loads, which would either eliminate or minimize any tensile loads, in this area. Tensile loads are normally the worst case for testing the head strength of a rivet (manufactured or shop).
Another reference also found in section 5D of the builder's manual pertaining to the subject of drilling out rivets is based on an excerpt from the Alcoa Aluminum Rivet Book, dated 1984. Without quoting the entire paragraph, the upshot of this reference is summarized in the following 3 points:
1. Poorly set and cracked rivet heads tested in tension were still within 5% of the strongest properly set rivets indicating that minor deviations (.007 inches) are not cause for concern or replacement.
2. A second rivet that is driven in any one hole is likely to be more defective than the first...and will be more likely to buckle and form an imperfect head.
3. Tests have shown that very small rivet heads are sufficient to develop the strength of the rivet shank, even when the rivets are subject to a straight tensile pull.
It is my conclusion that the risk to reward of drilling out and replacing 73 rivets, (a combination of AN470 and AN426 rivets) in the center section structure is absolutely not merited. There is sufficient evidence both from within and without the RV-12 community to support this position.
The previously cited 2 threads of relevant discussion on this subject can be found here:
1. http://www.vansairforce.com/community/showthread.php?t=44303&highlight=20-04
2. http://www.vansairforce.com/community/showthread.php?t=39106&highlight=20-04
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