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Just flipped my bike over, spotted this hole in the underside of my Tiagra derailleur. Looks like a screw should be in there, but there it is not. Should there be? Should I worry? What is this screw for/called?

If no, what purpose does this bike serve?

Note these are pics of the underside of the derailleur.

Tiagra rear derailleur 1 Tiagra rear derailleur 2

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3 Answers 3

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That hole is for the outer plate stopper pin, one component of part number 8 in the EV diagram linked in the other answer. It prevents the pulley cage from pulling out of the derailleur body.

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  • It’s like a set screw type thing, right? I remember seeing it mentioned in OSPW install instructions.
    – MaplePanda
    Commented Apr 30, 2021 at 22:30
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    @maplepanda it’s shaped kind of like a dog nosed set screw, except the unthreaded nose is a lot longer than any of those I could find with a quick google search. The whole set with an outer pulley cage plate included seems to be under $10 from the handful of online shops that carry Shimano small parts, which doesn’t seem too bad as long as one’s not paying shipping on just the one item.
    – Pisco
    Commented May 1, 2021 at 2:48
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I'm not sure if there are different Tiagra rear derailleurs, but if you look at the exploded view of the Tiagra RD-4700, I don't see any screws coming in from the underside (except for the bolt that clamps the cable), so I don't think you're missing anything.

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I had the same fear that a screw came out of my Tiagra RD or something. But watching a youtube video eased my fear and the content suggested that it might be an access for servicing and it is by design. The video is not exactly for Tiagra (but for 105), and it has the same 2mm access.

Hope this helps.

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