To flush or not to flush?

I would really like to use flush rivets on my airframe ... they look better, you get a bit more performance/fuel economy from less drag, and they supposedly yield a stronger joint. Also, I'm already tool'ed up for it.

But, the Sling 4 is designed around standard, dome-headed, blind rivets! What to do?

Well first of all let's read up on blind rivets:
  • Some of the things that differentiate them are:
    • Materials used which in turn affect shear and tensile strength, as well as corrosion resistance.
    • Their initial diameter, and the size of the hole they can fill.
    • Their grip range.
    • Their type, such as closed or open-ended, multi-grip, etc. etc. there are MANY MANY types of rivets from many vendors, making things far less straightforward than with a solid rivet.
  • Unlike solid rivets, they don't fill their hole as easily and flexibly, this means that for flush rivets, the final hole, post-dimpling, has to be reamed to the proper size, which in turn means that the initial pre-dimpling hole has to be slightly under-sized to account for the stretching the hole will suffer in the dimpling process. At least I think so ... I may be exaggerating the effect of the stretch ... This picture explains the issue well:

So what does this mean when trying to dimple and flush a rivet an aircraft not designed for it?
  • In theory, one should start with an undersized hole that can then be dimpled (and therefore stretched and enlarged) and reamed to final size. So, for a 3.2mm-1/8th rivet, you could start with a 2.4mm-3/32nds hole (for example), and dimple it with a 1/8th dimple die that has a 3/32nd pilot on it, such as this one sold by Cleaveland Tools. You then ream it to final size. Seems there other kits that work this way.
  • On the sling however, holes already come final sized for the planned standard domed rivets, so a 3.3mm hole for 3.2mm rivets in most if not all skins.
  • If I take a 1/8th dimple die (which I already have!) to such a hole, the hole will get further enlarged, and the hole could very well (likely?) be too large for the 1/8th blind rivets. I will have to test this and/or check with the factory, but I'm expecting the worst here. It will get stretched enough that I couldn't ream it to 1/8th for sure (since that's the size of the hole before dimpling!).
  • So, I could consider enlarging the holes, right? Drill an undersized hole for the next size up (4mm-5/32nds hole size), dimple, ream to 5/32nds (or #21), and voila! But not so fast. .... where does one find such a dimple? Well nowhere as it turns out. Cleaveland Tools seems to produce the only dimple for such an approach, but they do it for the smaller size only.
  • Sling's main rivet is the 3.2 x 8mm ... this is a multi grip rivet. This means that it may be used in various places with various grip requirements. Depending on the selection of flush rivet(s), I may need multiple sizes where TAF uses only one ... a slight inconvenience to keep on top of, but a decision factor.
  • Need to confirm grip requirements, possibly across multiple sheet thicknesses. 0.032 is two thin sheets together. A sheet + a rib would be 0.036 or more ... Dimpling will increase that, but I'm not sure by how much ... hopefully enough that  a rivet with grip of 0.63 or more will do, because that seems to be the common grip size starting point.

Things I don't think will work:
  • Dimple the factory provided hole with my 1/8th dimple and rivet, without reaming: poor grip on sharp edges (see image above). So far I have not seen anyone online suggest this would be an OK approach. Reaming to the next size up (#29) creates a hole too large for any 1/8th rivet I've seen.
  • Dimple to 1/8th, ream to 5/32nd and use a 4mm rivet. But the dimple would not be the right size for that rivet and it would sit proud. Defeats the purpose!

Things that could work:
  • Get the factory to provide the right rivets and dimples if they have them, They've built flush slings before, so clearly there must be a way I am not seeing.
  • Get custom dies done? Might be cost prohibitive. Also, all this is based on the idea that all skin rivets are the same size, which may or may not be true (otherwise, need multiple custom dies!|). Could also need bigger size(s) to "fix mistakes". No countersink cutters for 120 degree 1/8th rivets either ...
  • Dimple to 1/8th, ream to 5/32nds, and dimple again with a 5/32nds dimple and use 4mm rivets. There could still be a slight unevenness in the final hole from the second dimpling step, but maybe it's tolerable? Lots of work though.
  • Forget about this idea all together, save some money and time, and stick to the domed rivets!

And finally, some other factors:
  • Sourcing the right kind of rivet could be a challenge. For non-aerospace rivets, I've so far had a very hard time finding the sizes I expect I would like through retail outlets, or by anything other than multiples of hundreds (overkill for basic testing). CherryMAX rivets are more easily come buy, but they cost a fortune and are complete overkill for my needs, though bought in quantities, and shopped around for, I'd be willing to do it if I had to! There's also the Cherry MS rivets that would be fine for my needs and a little cheaper.
Whew! More complicated than originally thought. I will ponder all this and take this thoughts to TAF, see if they can help ...