It's an M4 socket cap screw. There's loads on ebay but like Jurek says, it's hard to tell when ordering if they're fully threaded.
The alternative is a grub screw, like what I have on my flyer. Something like this.
It's an M4 socket cap screw
Unless it's M3, like most dropout stop screws 🙂
Accu have long full-thread screws in hex head, if @amey isn't fussy about what's on the driving end, e.g M3×40
I was gonna say 'probably' but was confident I was right. In hindsight it does look pretty skinny.
@gbj_tester started
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It's an M4 socket cap screw. There's loads on ebay but like Jurek says, it's hard to tell when ordering if they're fully threaded.
The alternative is a grub screw, like what I have on my flyer. Something like this.