I had a few people ask me why I thought solid rivet bucking was easy. I have to admit that I was fortunate to be taught how to drive rivets in A&P school a fair number of years ago, but the skill could be easily picked up by anyone mechanically inclined. I had the opportunity to make many mistakes before getting it right. I have not riveted much since then so there was a little bit of refresher needed. If you have access to the EAA web site, there is a Tony Bingelis article about rivet driving in the homebuilders section. There are also a few tutorial videos on the EAA site about riveting. It is also covered in his books. I will give you a very condensed version that hopefully does not set anybody astray.

Obviously you need four basic tools: Air compressor, rivet gun, rivet set and a bucking bar. For our relatively small riveting requirement, find a local RV builder and borrow his stuff instead of buying. You will only need it for a few days. Here are the tools I borrowed.

Rivet guns are rated by 2X, 3X and 4X with the higher number being the stronger gun. 3X is adequate for both 5/32 and 3/16 AN470AD rivets. Try to get a rivet gun as opposed to an air hammer. An air hammer hits too hard and too fast and will only cause you aggravation.

Rivet sets for our domed rivets are cupped and have the size inscribed on the shank.

A bucking bar is any piece of steel that has a smooth flat side on it. I used a regular bucking bar on all of the side spar doubler rivets because access was not an issue. The rivets that attach the new top doubler to the seat angle required a thinner bucking bar than what I had. I just happened to have a piece of steel laying around the shop that served the purpose nicely. In this picture you can see the regular bucking bar as well as the small piece of steel that worked better under the top doubler.

I set my air compressor at 70 PSI and then used the adjustable knob on the rivet gun to get what I considered an adequate pressure. Set this with the rivet set attached to the gun and pressed against a piece of wood. NEVER squeeze the trigger without the gun and set pressed firmly on a surface. Otherwise you will be launching the rivet set across your shop. Practice will let you know if the pressure is set right. Use scrap aluminum and buy/borrow some extra rivets. Buck a bunch of rivets some with varying pressures until you get it right. The desire is to get to the point where you can set the rivet with one or two relatively short trigger squeezes of about a second or two in duration max. Using a lower pressure and milking the rivet over a long time will work-harden and possibly crack it.

Make sure to apply firm pressure on the rivet head or the set will bounce around and put a large “smile” mark on the rivet, or worse, dent the aluminum structure. Don’t avert your attention from the rivet gun side to the bucking bar side during the drive or you will surely slip. An over driven rivet can be easily drilled out, but a dented piece is harder to fix. Practice will let you know about how long is long enough to drive your rivet. Then you can look at the bucked end and see if it needs an extra few taps. I ended up “smiling” three rivets on my center spar and replaced them.

The bucked side of the rivet tolerances are specified in AC43-13 as well as Chris Hientz’s Zenith Construction Standards guide. Basically you are looking for a bucked head to have a diameter of 1.5 of the original shank and the height to be about 0.5 of the original diameter. They also give min and max tolerances. What I did was set my dial caliper to .234, which is 5/32 X 1.5. If the rivet head diameter was close to this, I stopped. Also look to see if you bent the rivet over sideways instead of flat. This is caused by not having the bucking bar held perpendicular. Here are some bucked rivets.

The only center spar unique problems I encountered had to do with where the web AN4 bolts lined up with the new top doubler to seat angle rivets. Luckily I had not tightened the nuts down because I had to remove them all to gain access for the bucking bar. One web lined up directly with a rivet and I had to get creative with the bucking bars to set the rivet. If I had seen this ahead of time, I would not have attached that web. Look up a few pictures to see how the webs will be very close to some rivet locations.

Lastly, if possible, keep it in the family and teach your children how to rivet.

Good luck and have a bucking good time!

Dave Gallagher