Clinching Dimpled Fuel Tank Rivets

[DanH]

Originally Posted by jcarne

Lol I hope that is sarcasm.

Not sarcasm.

If Tucker can't swell the shanks to fill the oversize holes, the assembly is structurally substandard, and prone to leakage and paint blisters. If he can, great, but it will require a longer rivet, and he is already having problem with the standard length.

[chaskuss]

Originally Posted by DanH

Not sarcasm.

If Tucker can't swell the shanks to fill the oversize holes, the assembly is structurally substandard, and prone to leakage and paint blisters. If he can, great, but it will require a longer rivet, and he is already having problem with the standard length.

Been there, done that, got the Tee shirt. I found that best solution was to use the next size longer rivets. Swell them slightly, using a hand squeezer, before installing the rivets in the structure. In other words, swell the rivets while they are simply mounted on the squeezer, but not in the structure. That way, you simply throw away any rivets that don't swell properly. Swell as needed to get a snug fit in the oversized holes. Install the swelled rivets into the holes and set as normal. Without the excessive rivet to hole clearance, your results will be much better.
My issue was not on the tanks, but on the curved portion of the turtle-deck.

Charlie K

[I.am.Human.Factors]

Thanks for all the helpful hints, tips, and encouragement everyone! You all had some excellent ideas! While there was a little doom and gloom among this thread, I tried the simple act of wiping the excess proseal off before setting and that did the trick nicely. Some good rivets were resulting so I’m much happier now.

I’ll definitely be drilling a number of of rivets out that I had previously set, but I wasn’t overconfident and set all of the ribs before posting this message. I set a rib and a half and stopped to think about it. I learned the lesson to slow down long ago.

Thanks again everyone!

[erikpmort]

I found the answer ! For me.....

Use the regular yoke not the longeron yoke. Folding over be gone ! Didn’t even have to clean them anymore

[DanH]

Tucker, forget doom and gloom. There is risk in allowing emotional content into what should always be a dispassionate build decision. Dispassionate is the key. If you're rationally sure of what you've done, great. If you're not, replace it. Either way, don't let the decision be swayed by feelings, yours or others.

Exactly half the finished RV's are below average.

Long term, no one has ever regretted replacing a marginal part.

[Mckay]

One of the issues is using B proseal to Fay seal ( also letting sealant cure before riveting). We never used B to do this, B is for filleting and A is the proper sealant for Fay sealing. We drove hundreds of thousands of wet rivets and never cleaned tooling or rivets until they became so gummed up it made more mess than progress. This is a non issue when using the proper sealant, I have no idea why Vans calls for B in this setting other than it is more forgiving in application than A.

I agree with the others, drill and/or replace parts, leaks suck. You’ll get there and be flying soon!

[jcarne]

Glad you got the problem figured out. Those dies from Cleaveland have worked for many a builder, no reason it shouldn't work for you too.

[Bill Boyd]

... just reflecting aloud...

I can't see where those flanges, properly fay-sealed, would allow fuel to reach the dimpled and riveted areas, hence not seeing the leak concern. But I know fuel tanks, leaks and paint blisters are fickle and mysterious things and that meticulous technique is never a bad thing.

2. Once cured, I wonder how much the rivets contribute to the mechanical strength of a ProSeal-bonded joint. I really do. Especially after trying to get a fuel tank inspection cover off that was ProSealed on years before. From what I experienced that week, I am skeptical that a few rivets, whether clinched over or beautifully set, will enhance that bond strength by much. But I am not an engineer, I'm not DanH, and won't pretend to be.

Carry on.

[DanH]

Originally Posted by Bill Boyd

... just reflecting aloud...

Reasonable reflections Bill.

First, understand there are no standards for polysulfide sealant in avgas. All standards are written for jet fuel. The test fluid is actually called JRF, Jet Reference Fluid. It's important to remember, because we don't know how the physical properties of the cured sealant are affected by 100LL, a more volatile compound. For example, no one I know has yet offered an explanation for sealant reversion...sealant turning back into goo.

Second, cured sealant is not a solid. It is full of tiny air bubbles, like a sponge, and thus cannot act like a hydraulic fluid under load.

Third, it is not vapor proof. It emits toluene for a few weeks, and will pass fuel vapor after cure.

Ok, the test standard closest to what we might expect in our application is a sample soaked in JRF for 14 days at 140F. The minimum performance is 50 psi and 100% elongation at failure. The test article from one lab report on my hard drive scored 128 psi and 600%.

Consider those numbers. They are terrific in the context of a rubbery sealant between two parts. However, if we are expecting that sealant to fixate parts as an adhesive, we would be greatly mistaken. Although 128 psi feels like a lot when we're trying to remove a cover plate, it is Jello pudding in the context of structural requirements. For comparison, ordinary West 105/206 is 7320 psi tensile and 11,500 compressive. 2024-T3 is 50,000 tensile yield, 70,000 ultimate.

So, it's not very strong. It's also not very stiff, per the elongation number. At 128 psi, the sample stretched to six times its original length. Clearly that's not possible in a fay joint, as it would have long since sheared all the rivets. It's like pairing a steel cable with a rubber band to pick up a weight. The rubber band sees almost no strain, and contributes nothing useful. The cable takes all the load.

In our tanks, the sealant essentially contributes zip. Load transfer is all via the riveted joint.

[ryankhallcpa@yahoo.com]

"Exactly half the finished RV's are below average."

After I got off VAF and returned to my day job as a bean counter I realized....haha he's exactly right!