Countersunk Rivet Notes

Posted On August 5, 2020

I’ve learned a few things as I tried to install countersunk rivets on my fuselage, which I’m capturing in this post. In summary, the rivets I originally started using (POP AK41H and AK51H) are not suitable for use on the fuselage, but I found an alternative part that’s readily available from McMaster-Carr.

Long ago I decided I wanted to install flush rivets in along the forward fuselage, to match the improvements on the Sling TSi. This is a slight aerodynamic improvement, but I also think it cleans up the appearance. Sling provides part # HW-RIV-253-X-X-1 for this purpose on the TSi, a 4.0x10mm countersunk rivet. I had procured an alternate rivet – POP AK51H, which appeared to be a good fit according to strength and grip length parameters. I bought these AK51H’s because I only had a handful of ‘253 rivets, and did not know how to get more.

What I learned is that the shape of the Stanley POP AK51H rivet is quite different. First, the AK51H is a closed-end rivet, whereas open-end rivets are used almost exclusively on this plane (fuel tanks being the exception). The other, more important difference is the shape of the factory head: the diameter is larger than a typical 5/32″ rivet, and is much larger than the ‘253 rivets. I measured ~8mm on the AK51H rivets, vs. 6.4mm on the ‘253 rivets. You can see the difference in the following photos.

There are two negative consequences of the large head on the AK51H rivet. First, the rivet sits proud of the dimple (i.e. above the skin), rather than flush with the skin as shown in the next photo.

The second consequence I learned when I tried to drive in the AK51H rivets: they were causing the skin to “pillow”, creating an ugly distortion. I believe this is because the rivet was applying clamping force at the edge of the dimple, rather than in the valley of the dimple as required to properly clamp the two pieces. The only way I could think of to fix this would be to make a larger dimple. To my knowledge, no dimple die exists that would make a larger 5/32″ dimple, and I didn’t want to try to invent one. I was about ready to give up on the countersunk rivets, which would have also meant scrapping the side skin I already dimpled.

A fix to my problem came about by accident. A few weeks ago I was carrying out Service Bulletin 0017. I purchased a few rivets from McMaster, including 5/32″ (4.0mm) rivets that I though I might need to replace drilled-out rivets from the seat rails. I wound up not needing them, but I stumbled upon a happy coincidence: these rivets – item 97530A618 – appear to be identical to the 4.0x10mm rivets that Sling furnishes as part HW-RIV-253. The dimensions are identical. McMaster doesn’t say on their website who manufactures their rivets, but the packaging reveals that they are an Avdel AVEX® rivet, part 01604-00514. I had assumed that Gesipa countersunk rivets were supplied by Sling, since most of the other rivets are made by Gesipa, but that doesn’t appear to be the case for these countersunk rivets.

Once I confirmed these rivets fit well, I ordered 6 more bags (of 50 pieces) from McMaster at about $8 a bag, not too bad. As expected, these rivets went in nicely and I was able to complete the side skin install. I lightly abraded the skin along the rivet line (like the factory does) to promote paint adhesion, just before installing the rivets.

I’m very happy to have found these rivets, it looks much better and makes my aircraft more consistent with the factory design. I still have to install the other side skin before I can move on to the undercarriage.