These special fasteners allow you to joint tubes together using screws instead of plastic rivets. This is a stronger joint, and it great for holding the e-bay in your rocket. Specially treated so that epoxy will stick permanently to the aluminum inserts.
There was a time when you could buy them straight from the manufacturer and I got a whole bunch of them in bulk. Now, it appears you can only get them from Apogee. Too bad, they are pretty slick and Apogee is selling them at a premium.
Once my current supply is gone, I am not sure I will buy them at $5 a pair. I have been looking around for a replacement, but have not found one yet.
I'm assuming by now you placed the order. But if for some reason you have extras i'll give some a try since i'm almost out of rivets. Current project is 3" so I guess some 4/40? And whatever the next smaller size is for a 54mm.
Delivered free in 2 days, they seem perfect enough for me, 'cept you can select 3 different 'thicknesses' of the toothed, grippy-sections, and I went with the thinnest one. No biggie, but I'd recommend you go with the medium-size