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Doing front rotors, pads, and wheel bearings on my Subaru recently I had all four of the 17 mm bolts that attach the caliper brackets to the 'knuckles' break off. These snapped off cleanly and felt like they were all breaking with roughly the same torque. I had removed them with a breaker bar & socket.

Looking at the sheared metal just afterwards I noticed the same pattern of rust-like coloration on a portion of the exposed surface on all the bolts. The photo below shows the bolt head; the red circled area is mainly what I'm referring to, but the green area might be more of the same.

This discoloration was visible immediately after the bolt broke; but I don't think it was "flash rust" occurring shortly afterwards, mainly because its not uniform.

I don't think there were any other contaminants that could have been introduced either to give the appearance of rust.

So what does this mean? Were these bolts failing before I even touched them? i.e., was this a crack? And if so, what might be the cause?

enter image description here

I had asked one earlier question related to this: How to remove broken bolt stubs from brake caliper bracket?

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  • See mechanics.stackexchange.com/a/80612/10976 and do us all a favor and accept the answer.
    – Solar Mike
    Dec 18 '20 at 6:09
  • The fact that you had to use a breaker bar to literally break these bolts tells me that they were fine before you over-torqued them. That is always a risk that you take when you use such a tool.
    – jwh20
    Dec 18 '20 at 10:06
  • @jwh20 they broke before they turned at all... I'm not sure what another option was. I had actually applied penetrating oil on the first pair which didn't help, and I was advised not to try to use heat to break them loose. Dec 18 '20 at 11:06
  • Heat is often effective as are products like PB Blaster or other penetrating oils. But just because they didn't move doesn't mean you didn't over-torque them. In cases like this you may not have any option but to break them. But it seems like you are trying to blame the bolts. The fact that they took way more torque than their design limits before breaking tells me they were ok apart from the rust.
    – jwh20
    Dec 18 '20 at 11:31
  • @jwh20 I'm just interested in why the metal has the appearance it does. Maybe I could have done something different, that's almost certain :) Dec 18 '20 at 11:33

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