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Bolt size / frame thread pitch for rear mech protector cage?

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Quick question. Does anyone know the bolt size / thread pitch for those attachment points on kids' bikes? I have one of those rear mech protector cages to put on a bike but it didn't come with bolts.


 
Posted : 24/09/2025 1:31 pm
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M4, usually 


 
Posted : 24/09/2025 2:27 pm
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I would say 5mm, the standard pitch that you can get from Screwfix. It would be unusual for it to be 4mm as this is one of the few standardised standards in the bike industry!


 
Posted : 24/09/2025 3:06 pm
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Bottle cage bolts are usually M5, do you have one of those you can whip out and try in the hole for size?


 
Posted : 24/09/2025 3:18 pm
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@devash if you tell me what length you require (by private message) I can drop a couple of stainless in the post to you. Free.


 
Posted : 24/09/2025 3:57 pm
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Thanks all, so I'm guessing standard 0.8 pitch like water bottle bolts. Will check the bike at the weekend (it's currently at a relatives) and see if M4 or M5.  @wheelsonfire1 many thanks for the kind offer but I should have some suitable ones lying around if they're just standard bolts.


 
Posted : 24/09/2025 7:19 pm
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M5 is pretty well the standard on bikes for things like bottle cages, mudguard and rack mounts having assembled and built-up hundreds of them - the only time I’ve seen anything different is for drop-outs/mech hangers which are more proprietary.


 
Posted : 24/09/2025 8:19 pm
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 My guess would be M5x0.8, but those are for bolting on guards/racks. 

Mech protectors tend to bolt onto the axle. Eg: 

IMG_0647.jpeg


 
Posted : 28/09/2025 9:12 am
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Fitted yesterday. Standard M5x0.8 / water bottle bolts as everyone mentioned. Thanks all for the info.


 
Posted : 29/09/2025 1:44 pm