15

enter image description here

Is this normal? I just noticed it when I was cleaning my bike after a long day downhill mtb. Are those screws? Thanks for any replies.

  • 3
    My best guess is that the bolts are hollow, and what you see are the bits left over from gouging out the hex "sockets". Likely were there when you bought it. – Daniel R Hicks Apr 25 '17 at 11:32
  • 1
    Agree, this is a pretty common thing to see. Take one out and poke at it and you should see that it's just a little nonstructural flake of material. – Nathan Knutson Apr 25 '17 at 19:53
  • @tankerBeast I suggest you pick the answer that best matches your findings. My answer is probably incorrect and doesn't deserve to be the accepted one. What are your findings ? – Criggie Apr 26 '17 at 8:28
26

Are you sure those hex bolts are cracked?

They could instead just have peeling chrome. Hex bolts tend to accumulate water and grime and thus corrosion. It could just be surface peeling of the chrome.

I'd poke at them with a sharp ice pick and see if it's just surface peeling of chrome or if the bolts are actually cracked. If it's the former you can ignore. If it's the latter, follow @criggie's suggestion.

  • 1
    The placement of the damage does seem very odd, flaking would be the simplest explanation. – Rider_X Apr 26 '17 at 17:01
13

The good news is those bolts aren't load bearing parts directly. The bad news is those are holding the load-bearing parts in place, so a failure of the bolt will allow the pivots to fail very quickly.

I'd strongly recommend taking them out and getting replacements from the maker of your frame.

Have a look at the answers to this question about shearing rotor bolts notice how little beef there is between the head and the shaft? Damage to bolt heads needs to be fixed.

5

Take them out and check if they are hollow, it might be that those are only the burrs from machining the hex.

  • 2
    You raise an excellent point, but the answer a bit short. Could you please used Edit to expand the answer - perhaps why you think the bolts might be hollow ? How would the rider check the bolts for hollowness by taking them out ? – Criggie Apr 26 '17 at 8:05
3

Those are artifacts or chips resulting from broaching the hex into the bolt head. Should be nothing to worry about but ask a mechanic if you want to be sure.

3

That's left over material from the broaching of the hex head in the bolt.

broaching is a machining process in which material is removed. In this case the inside of the hex is formed with a broaching tool

  • Can you please expand on "broaching" ? I don't recognise the word in this context. Use the Edit link to grow your answer with more details. – Criggie Apr 26 '17 at 8:02
2

Broaching in this reference: Hammering a hex shaped tool into the round hole in the bolt (maybe done when the bolt is red hot) to produce the hex profile for the allen wrench. Pushes the material from the sides of the hole down and leaves the strange looking ragged chips you see when the tool is pulled out. The aluminum surrounding such a hi-tensile bolt would probably fail long before the bolt fails.

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