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Roll tap for thread repair?

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  • #16
    well even a blind squirrel finds a nut every now and then I'm not well versed in BMXes, though I have heard of Profile. From my mtb cranks that ride on splined bottom bracket axles, the bolts at the end are for pinching the 2 crank arms together = bearing preload. The crank arms themselves are held on by pinch bolts that squeeze each arm on the splined section. If that's the case with these you could probably use a piece of hardened chewing gum in that hole to provide bearing preload. On my bikes that have that style crank the "bolt" is plastic and the tool that you finger tighten it with is also plastic. I'd double check, but as long as you can get a bolt started and it takes, it may not matter all that much.

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    • #17
      Hi guys. Just wanted to report back: mission accomplished. Tap turned up this morning. Procrastinated what I ought to be doing for work and ran it down the "good" thread. Enough resistance from rust and crud to make the whole part warm - even with cutting oil.
      Couldn't get it aligned with the thread on the mangled side - at least not so as I could be confident that I wasn't just making it worse - so I bored out about 5mm of damaged threads. Used a small bar with an aluminium profile CCGT insert - it was already loaded and we're talking light cuts so I thought it was worth a try. Worked fine and I used a drill bit to just chamfer the lead in. Then I could get the tap in but it was still a bit hope and pray.

      Incidentally, it was hard enough that trying to use a zero flute countersink to clean up the start of the hole (where you could see the start of the M10 thread) did nothing. Top slide at 45° with the boring bar cleaned it up nicely though.

      Thanks for the advise guys. Much appreciated.

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      • #18
        nice, glad to hear you got it sorted, add another feather to your cap

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        • #19
          Why not pick up the thread with a boring bar and chase it out on the lathe ? ? ?
          Set an indicator stop so you don't crash it, but it is far from an impossible task.
          You guys have lathes, right ?

          -D
          DZER

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          • #20
            You seem to be saying that the bolts that are held in those holes are not in any way heavily loaded or responsible for ANY safety concern. If that is the case, I say go ahead. But if there is a significant load, perhaps indicated by a required torque value, then it would be nothing short of complete folly to even attempt the repair of that part.



            Originally posted by mattthemuppet View Post
            well even a blind squirrel finds a nut every now and then I'm not well versed in BMXes, though I have heard of Profile. From my mtb cranks that ride on splined bottom bracket axles, the bolts at the end are for pinching the 2 crank arms together = bearing preload. The crank arms themselves are held on by pinch bolts that squeeze each arm on the splined section. If that's the case with these you could probably use a piece of hardened chewing gum in that hole to provide bearing preload. On my bikes that have that style crank the "bolt" is plastic and the tool that you finger tighten it with is also plastic. I'd double check, but as long as you can get a bolt started and it takes, it may not matter all that much.
            Paul A.
            SE Texas

            And if you look REAL close at an analog signal,
            You will find that it has discrete steps.

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            • #21
              well, now that I've looked up Profile BMX cranks I'm not sure. If you can use longer bolts I think you're all good though.

              BMX cranks
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              mtb cranks (one type at least)
              Click image for larger version

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ID:	1940683

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              • #22
                Doozer I was thinking that might be something to try but I've not got threading tooling small enough to get in there. Also, I have a metric leadscrew so the error in creating an imperial thread would probably be fine cutting a new thread but I fear it might make it too sloppy. Done now though.

                Paul Alciatore That's exactly the case. In theory this bike is going to be a show bike and not ridden. However I'm not willing to do a job that looks good and waits to kill someone once the fact it's only cosmetic has been forgotten. That said, this bolt doesn't take any load as far as I've been made aware. Apparently some are even made from plastic. The load is taken by the splines and the cranks are pulled onto those using this thread. I've not removed any of the meat so haven't weakened the shaft.

                Got the silver washers to make next that fit round the CSK head screws you can see in Matt's picture. Might sound a daft thing to make but for four of them (two sets) they want the best part of $100.

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                • #23
                  If your not sure on the thread size or if it will hold then drill it straight through and put a piece of threaded rod through the shaft with a nut on each end.

                  JL...............

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