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I've just noticed this screw is missing from the derailleur mount on my bicycle. I don't know how long it has been gone, or how it fell out, as I only noticed it when I had remove the rear wheel. It looks like an M8 so I bought an M8 screw and it almost fits. The screw goes in a half turn or so then stops. So it's about the same diameter as an M8 but the thread pitch must be different (do M8 screws come with different thread pitches?).

The bicycle is a Pinnacle Lithium One about three years old so it's very unlikely to be an Imperial screw. But that leaves me stumped as I can't think what alternatives to an M8 there are. If anyone can suggest what screw is needed I'd be most grateful.

Later: the M8 bolt I tried has "TD" stamped on the head and I believe this means it is a standard pitch of 1.25mm. I have no way of measuring the pitch in the hole on my bicycle but it looks finer than the screw.

Missing screw

This is an attempt to photograph the bolt right next to the hole so the threads can be compared. It does look as if the threads in the hole are slightly finer than the bolt.

Thread pitch

2 Answers 2

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M8×1 exists; it's the most common fine pitch M8.

Whether it's used for your RD hanger is another question. Some apparently use M8×1.2, which is pointlessly close to the standard 1.25mm pitch, and M8×0.75, which is extra fine.

A thread gauge would be a good idea at this point, as internal threads are hard to measure by the alternative of measuring several with calipers and dividing. However M6 has a pitch of 1mm and will drop in to the thread. You slide it out gently, and all the threads will rise and fall at the same time if your female thread is a 1mm pitch.

I wouldn't expect a 1.2mm pitch to look finer by eye, and a 0.75mm pitch would look much finer.

You might get some help from a supplier of third party hangers, like https://www.rearmechhanger.com - they carry a hanger for the disc version of the Lithium and may be able to tell you what thread that uses; some of their hangers come with mounting bolts but it's not clear all do

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Chris correctly identified the thread as M8 × 1mm but I'm adding this answer because it turns out there is more to it. I'll post the details in case it's of use to someone in the future.

A standard M8 × 1mm won't work because the head is too large. The screw is a custom part only shipped by Shimano as part of the hanger kit (though I imagine you could find it on Ali Express). I know this because I took the bicycle to my friendly bike shop and they (a) told me the above, (b) rummaged through their boxes of used parts and managed to find a matching screw, and (c) didn't even charge me!

So colour me pleased - my bike is fixed, and and for free. Support your local bicycle shop!

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