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  1. #1
    Senior Member Binxsy's Avatar
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    Reccesed allen head bolt question.

    I know this is a rather mundane and maybe stupid question but anyway.

    I picked up some used ultegra sl brakes and the recessed allen bolts are freakin long. Like 30mm long and they aren't very nice looking too.

    Anyway I am mounting there on a 89 peugeot triathlon. The current bolts are a snug fit I am guessing that the mounting holes for brakes have been a pretty static size. I measured the length of the hole and it is 15mm(pretty sure). On loosescrews.com there is a 10mm recessed bolt. Will this 5mm make a difference or what???
    So It Goes.....

    89 Modernized Peugeot Triathalon
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  2. #2
    bike whisperer Kimmo's Avatar
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    I'm not sure what you're asking... you're referring to the tubular nuts that secure the brakes the the frame and fork, right?

    AFAIK they're a uniform size; required length may occasionally vary a bit though... although the 30mm ones you mention sound like they're to enable fitting a rear brake to the front or something...

    I'd say the bare minimum engagement for this fastener would be no less than three or four threads...

  3. #3
    Bianchi Goddess Bianchigirll's Avatar
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    Hi Binxsy it sounds like the mounting bolts you got are for a carbon frame and fork. I do not think the 5mm will make a big difference.
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  4. #4
    Senior Member
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    Recessed mounting nuts (They are nuts, not bolts. The "bolt" is part of the caliper assembly.) come in a variety of lengths from 10 to 30 mm to fit forks with differing crown thicknesses and rear mounting. As long as you have adequate thread engagement (as noted at least 3 or 4 full turns and 5 or 6 are better) you should be ok.

    Here is a source for them:
    http://www.loosescrews.com/index.cgi...s&id=252707975

  5. #5
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    Quote Originally Posted by HillRider View Post
    Recessed mounting nuts (They are nuts, not bolts. The "bolt" is part of the caliper assembly.) come in a variety of lengths from 10 to 30 mm to fit forks with differing crown thicknesses and rear mounting. As long as you have adequate thread engagement (as noted at least 3 or 4 full turns and 5 or 6 are better) you should be ok.

    Here is a source for them:
    http://www.loosescrews.com/index.cgi...s&id=252707975
    Hill rider has it almost right.

    recessed mounting nuts come in various lengths to reach the bolt on standard brakes with forks of varying thicknesses. It's a lot easier to do it this way than to provide different mounting bolts on the brakes.

    One not so minor point I disagree with.
    Screw threads don't reach maximum strength until the engagement depth equals about 75% of the diameter of the screw*, so for your 6x1 brake mounting bolt you want a minimum engagement of 5 full threads or 5 or more turns of the nut if at all possible. If it's on the borderline, use a longer rather than shorter nut provided it doesn't bottom out.

    BTW- if yours is too long (bottoms out before it's tight) you can cut it down with a hacksaw. The cut edge will be inside the fork hidden from view.

    * source click here
    Last edited by FBinNY; 01-02-10 at 11:07 AM.
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  6. #6
    Senior Member Binxsy's Avatar
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    Thanks! I think I might just cut the long ones I have. But yep thanks for the info!
    So It Goes.....

    89 Modernized Peugeot Triathalon
    79 Peugeot U-08 single speed
    8? Sekai Grand Tour 2500 single

  7. #7
    Senior Member
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    Quote Originally Posted by FBinNY View Post
    Hill rider has it almost right.....

    One not so minor point I disagree with.
    Screw threads don't reach maximum strength until the engagement depth equals about 75% of the diameter of the screw*, so for your 6x1 brake mounting bolt you want a minimum engagement of 5 full threads or 5 or more turns of the nut if at all possible. If it's on the borderline, use a longer rather than shorter nut provided it doesn't bottom out.
    Yeah, right after I posted I did the calculation and realized a minimum of 5 turns are needed to get adequate engagement on an M6x1 bolt and 6 full turns are needed to get 100% of the bolt diameter inside the nut. So my "5 or 6 are better" should have been more certain and said 5 or 6 full turns are required.

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