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Engineering / Metal Shop Advise Please?

comsensecomsense Posts: 245
edited June 2016 in Workshop
I broke a disc caliper bolt in a fork.

Drilled it out and tapped it. It is now M10 where it should be M8. Job is right so far..............

But caliper mounting bolts are M8. So, here is my question.

I have drilled and tapped down the centre of an M10 to make an "Insert adaptor" converting M10 back to M8.

Thickness of insert is now 1.8mm and I am wondering if this will be strong enough or should I abandon the entire idea for safety reasons? Anyone know? Am i being stupid?

Posts

  • lesfirthlesfirth Posts: 1,207
    comsense wrote:
    I broke a disc caliper bolt in a fork.

    Drilled it out and tapped it. It is now M10 where it should be M8. Job is right so far..............

    But caliper mounting bolts are M8. So, here is my question.

    I have drilled and tapped down the centre of an M10 to make an "Insert adaptor" converting M10 back to M8.

    Thickness of insert is now 1.8mm and I am wondering if this will be strong enough or should I abandon the entire idea for safety reasons? Anyone know? Am i being stupid?

    In principle I think what you are doing is OK. If you have done what you have you must have some expertise. If the mounting bolt will tighten to the correct torque I do not see a problem. Whether I am qualified to advise you or not is a matter of opinion. :)
  • stanthomasstanthomas Posts: 265
    If you can drill out a 10mm threaded bar with an M8 internal then you're clearly quite capable so, at the risk of teaching my grandmother to suck eggs:
    the conventional approach to thread repair is a heli-coil style insert (they look like a spring). For an M8 original you will drill out to 8.3mm (or 21/64 inch) then tap to accept the insert which has outside diameter of 9.9mm. Which is pretty much the same as the OD of M10.

    Whether this is safe depends on how much metal you have around the fork boss. If there's plenty then an insert will be stronger than the original. At a very rough estimate, I would say that you should have at least 2mm wall thickness. And the suitablity of your insert depends on what you made it from and how good you are at drilling concentric holes.

    The alternative, of course, is to fill the stripped thread with weld and re-tap.
  • redveeredvee Posts: 11,921
    What the OP is trying to do is make a helicoil which are used frequently but I guess the OD compared to the ID is much more than what he's suggesting.
    I've added a signature to prove it is still possible.
  • crankycrankcrankycrank Posts: 1,830
    If it were mine I'd just run it and check the tightness of the bolts after each of the first few rides. If OK then I would feel safe using this as a permanent repair. There will probably be some chiming in predicting suicide but use some common sense and don't plan a decent down Everest on your first ride. You should be fine.
  • jermasjermas Posts: 484
    I think I'd scrap it (safety first). I'm not sure enough of the original metal would be left to be 100%. Also every time you brake, all you'll think about is -I hope my repair is strong enough!
    If you do feel it'll be a good repair, use a high strength thread lock to permanently fix the insert.
  • homers_doublehomers_double Posts: 6,945
    I'd have gone down the helicoil route first but it sounds like a reasonable repair and unless you're mount is wafer thin I doubt there will be any problems.

    It's only one of the two bolts anyway...
    Advocate of disc brakes.
  • 6wheels6wheels Posts: 411
    As you've modified the fork leg to M10, I'd machine the caliper hole out to 10mm. The caliper(if at the rear of fork leg) is in compression when braking so not a lot of strain on the bolts.
  • svettysvetty Posts: 1,904
    Assuming the M10 'insert' is thread-locked in place and the M8 mounting bolt tightens without the insert shifting then you're likely fine. Just keep an eye on it for the first few rides
    FFS! Harden up and grow a pair :D
  • comsensecomsense Posts: 245
    6wheels wrote:
    As you've modified the fork leg to M10, I'd machine the caliper hole out to 10mm. The caliper(if at the rear of fork leg) is in compression when braking so not a lot of strain on the bolts.

    That is exactly what I have up to now and even though I've made the insert I will think it over before I go any further.
    I have a HY/RD caliper I want to use but don't want to drill it out to M10.

    Thanks Everyone, I do appreciate all the advise
    and I now have a better idea of helicoils and how to measure them. I have never had any education in these matters and only picked up what I know as I went along working on the bikes and cars etc. and we all now a little knowledge can put you over ditch :shock:

    Think at this stage I have narrowed it down to continue with M10 bolt and widened caliper or just shop around for a new fork.

    Thanks again for your informed replies.
  • fudgeyfudgey Posts: 854
    i can see no reason why that would not work, but quite a bit of effort.
    as said, i would have used loctite on the insert part (ok i would have helicoiled first but you didnt know about that at the time.)
    My winter bike is exactly the same as my summer bike,,, but dirty...
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