Research into Flush Rivets and a Nice Workshop Upgrade

Posted On October 8, 2019

EDIT 8/5/2020: the AK41H rivets identified below won’t work. See my later post regarding the side skin install.

Last winter my garage was far too cold to work in, especially during January and February, and the garage doors were too leaky to justify heating the space, which is about 500 sq ft. For years I had wanted to replace the ugly contractor-grade particleboard doors the house came with, so my airplane project finally gave us a good reason to install new insulated steel doors. They look great!

I didn’t spend much actual time working on the plane yesterday, because I spent my evening researching what it would take to put flush rivets onto the fuselage. I’d like to do it mostly for appearance’s sake, but it will also help keep the drag counts down. The Sling TSi uses flush rivets on most of the forward-facing skins. At one point, flush rivets were offered as an option on factory-build Sling 4’s but it was a costly option, $2750 – $7500 depending on how far they went.

I know it can be a lot of work to dimple and/or countersink the skins for flush rivets, so for now I’m just looking to put in flush rivets for 3.2mm holes in places that are easy to access – specifically the fuselage side skins, where they get stringers nailed to the back of them.

I did some research into the rivets this kit uses – from others, I knew the rivets were made by Gesipa, but a little extra digging led me to the actual part number used for the 3.2 x 8mm rivets – they are Poly-Grip blind rivets, part number GSMD41-43APG, which is also used on Vans RV-12. This part number is a trade size identifier, its decoded meaning is (source):

• G = Gesipa brand
• SM = Steel mandrel
• D = Dome style head
• 41-43 = rivet diameter & grip length. The 4 refers to rivet diameter in 32nds of an inch, so 4/32 = 1/8″. The 1 and 3 refer to the max grip length in 16ths of an inch; given this is a multi-grip style, it can grip materials that are between 1/16 and 3/16″ thick, which is not quite accurate; per Gesipa’s website, this rivet actually accommodates thicknesses between 0.5 – 5.0mm
• APG = per the guide I found, the characters after the sizing information are “proprietary”, with no standardized guidance on usage, but from context I can infer it means A = aluminum body, and PG = polygrip style

Gesipa also makes the same style rivet with a flush head, part number GSMC41-43APG (where “C” = countersunk). The grip range is somewhat smaller (1.0 – 3.0mm) but is otherwise identical in specs. As much as I’d like to just use this part and move on, I’ve been unable to find a distributor in the US. I know they exist, because an RV-12 builder managed to buy some back in 2012. So far, a call and email into Gesipa USA have gone unanswered, and the one distributor that replied to my email said they could special order them, but there would be a min buy of 25,000 pieces. Because of this, I started looking for alternatives from other manufacturers.

In the 3.2mm size, the Gesipa Poly-Grip rivets are rated for a typical strength of 720N in shear, and 1050N in tension, so I needed to find a rivet that is at least as strong, since I don’t know how much margin went into the aircraft design. There are other manufacturers of multi-grip rivets, but I couldn’t find one that made a rivet with equivalent strength. Manufacturers I looked into included Marson, Fabory, Jay-Cee*, Apex, McMaster* and Stanley/POP/Avdel (Stanley bought POP and Avdel, but still keep their products as separate lines). The * means that the vendor appears to offer private-label products that may be made by someone else, but I’m not sure. The closest options I could find in countersunk multi-grip rivets were about 5-10% less than the rated strength of the Gesipa rivets. Kudos to Gesipa I guess for having the superior product!

Since I couldn’t find a comparable multi-grip rivet, I looked into single-grip-length rivets, since they tend to be stronger. Most of the rivets I’m looking to replace on the side skin will need to be a 41-size based on my analysis: the main fuselage side skin is 0.64mm thick, and the stringer channels that attach to it are either 0.51 or 0.64mm, so the total grip length needed for a large number of rivets is 1.15 – 1.28mm, or 0.045 – 0.050″. The typical grip range for 41-size rivets is 0.031 – 0.062″, so that fits.

41-size rivets are very rare it seems — the only ones I could find were of the closed-end variety. POP makes an AK41H closed rivet with a rated strength of 305 lbf shear (1357N) and 385 lbf tensile (1712N), which is 1.9x/1.6x stronger than the Gesipa rivet, so I think this will work. POP brand rivets are also readily available from multiple distributors here in the US.

I have a 120-degree countersink tool from Aircraft Spruce that works using a standard rivet puller and common nails. I will buy a few sample rivets and give this a try on some test pieces and post my results.

One last thing – on Craig M.’s advice, I put in an order for the finishing kit so that I would have all the pushrods to install in the fuselage now while the skins are off. I’ve seen from the Sling FaceBook group that TAF is facing a huge orders backlog right now due to the popularity of the TSi model, so I expected it might take a few weeks/months. But surprisingly they had a finishing kit in-stock, and it will be here Friday! Time to make more room for all the stuff that’s arriving!