3.5mm hex on SRAM flat mount?!

[EB]

Edit- I’m an idiot

[Velocipede]

Cheap bolt. I've seen this on a few other SRAM parts as well as some other brands. Make sure you have a really good square Allen key unless you're planning on replacing it.

[oliver1850]

Harbor Freight has an inexpensive set of L wrenches with several half sizes:

https://www.harborfreight.com/hand-tools/screwdrivers-nutdrivers/hex-wrenches/13-piece-metric-ball-end-hex-key-set-96416.html

[EB]

Turns out it was a T25. Like I said I’m an idiot. All good now