I think they are OK
 

The average shop head thickness is (.033+.045)/2 = .039 so I would consider them OK, as others have also said. I've seen worse looking on flying aircraft... yours are better.

Short answer is, it is acceptable (not seeing the rivet). Rivet has no other defect? One or few rivet or many rivets? Is the rivet shop head centered over hole? (hard to determine sometimes) Cracks in rivet? (Get light and magnification).

It sounds OK. A stiffener on thin sheet metal is not loaded but is subject to vibration and thin skin flexing. Worst case is (after lots of flight hours) a rivet can get loose and "smoke". Will these smoke? Unlikely, but the best prevention is a perfectly driven (or slightly over driven) rivet.

You risk more drilling out the rivet. Structurally as you point out it is a stiffener. Set a standard and stick to it. Takes the head scratching out of it.

If you have not drilled rivets out in thin sheet metal make some practice parts and have fun. You know to only drill head off and try and punch the rest out.

Really up to you.

How do I fix this?
 

No matter what I do the rivet seems to tilt to the inside of the stiffener. I have tried different pressures, tilting the back rivet set slightly outward, two different back rivet sets (big black one and small Cleveland one). I didn't have this issue during the sports air workshop but I wasn't back riveting a stiffener. I attached some additional photos of the same stiffener to see if I can get any ideas to correct the issue prior to continuing. Thanks for all the input!

https://eaabuilderslog.org/?blprojen...cat=^empennage

Couple of ideas
 

Not sure exactly what is causing it, it might be due to some sort of alignment issue rather than the rivet set... though I had no joy with a spring loaded back rivet set, it tended to slip. I had more luck with a set similar to this, although mine has a rebate on the face:
https://www.browntool.com/Listview/t...6/Default.aspx

Suggested things to check:
- are the dimples formed consistently and not lop-sided before riveting;
- are you riveting from the inside-out (i.e. starting with the centre rivet and working out towards the ends of the stiffener);
- is everything fully clecoed before riveting;
- is the stiffener pressed up against the skin before riveting (use your fingers either side of the set to press it down);

Also, what does the factory head side of the rivet look like on the skin, is it nice and flush? If not, then the dimples may not be deep enough. If you could get another builder to come look over your shoulder while you work I'm sure there will be a solution, though what you have done looks quite acceptable.

Ya those look similar to the rudder I built. I found later in the build that if I took the guard completely off my back rivet set and use my fingers to hold the tip of it on the rivet I got some really nice looking rivets. This method is not without warning though, there is nothing but your fingers holding the set on top of the rivet and not slamming into your part. It works very well for me. Go slow at first and you can correct any clinching before the rivet is too far set. When I used the set with the guard I never got perfect results.

Bottom line though, them rivets look pretty good and I sure would feel comfortable flying in front of them.

I found that the guard did not let you get close enough to the centre of the rivet because it was interfering with the stiffer. I ended up grinding the flat edge more to remove the interference.

The face of many of the shop heads are not parallel to the skin surface.
The only way that can happen is if the rivet set was not perpendicular to the skin as you finished setting the rivet.
If that was the case, then there is a good chance it wasn't perpendicular as you started setting the rivet. That will make the shop head form off to the side slightly (the amount depends on how far out of square the rivet set was).

All of the rivets are more than acceptable and should be left. Just work towards improving as you go.

As already mentioned, sometimes the guard interferes with the stiffener slightly cause you to tilt the rivet set. Grinding a bit more off the flat with a belt sander can help that.

Btw, it really isn't a guard (but that is what people typically call it)m It's primary purpose is to apply a down force to the parts being riveted to make sure they are tight together as the rivet begins to set. As a secondary purpose it is there for insurance to prevent the set from accidentally wandering off of the rivet. If the tool is being used properly, that should never be able to happen though.

A common error (even by Flight Chops :o) is to operate the gun using one hand and expecting the guard to keep the rivet set in the proper position. This is a recipe for disaster.
There should always be one hand on the grip of the gun, and fingers of the other hand grasping the rivet set at the bottom. If I am using one with a guard, I just grab the guard. If no guard, grab the rivet set itself
It can not hurt you as long as you always keep the set tight against the rivet.
With practice, you can learn to put down pressure on the part right adjacent to the rivet set (a good practice to assure that the parts are in tight contact with each other) using your ring and middle fingers, and hold the rivet set with your thumb and pointer finger.

An additional tip that can help people having trouble aligning the rivet set square to the skin, is to place something right beside the rivet being set to use as a sight guide in determining if they are aligned (a small machinist square, precisely cut wood block etc.). Make the rivet set parallel to the corner edge of the object being use and you will be square every time.