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Roll tap for thread repair?

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  • Roll tap for thread repair?

    I've been given a bike axle to fix. It's essentially a 3/4" rod with a spline and a 3/8-24 tapped hole at each end.
    One of these (perhaps both) has had someone try to screw an M10 bolt into it and then someone try to sort it with an M10 tap. I believe neither succeeded in going very far. I originally suggested helicoils but that idea was rejected. I was then thinking of drilling it out and Loctiting in a cylindrical insert and tapping that. Now I have the actual piece, it doesn't look that damaged so I was thinking of getting a tap of the right size and running that down it. I spotted the option of a roll tap in that size and was wondering whether that might be any good for reforming the metal into the right place rather than just cutting it out. I believe this piece is in 4140 which I think would be difficult for a new hole but the thread is mostly formed already.
    Anyone know if this is a good/bad/terrible idea?

  • #2
    Maybe drill the hole a bit deeper and run the right size tap in. You should be able to source a longer bolt easily which will then have some new threads to hold into.


    • #3
      Is this a unique spindle? It sounds as if you'd be better off just buying a new spindle, if that's possible. May be cheaper than buying a roll tap, 100% chance of you ending up with a good spindle instead of one with half a broken roll tap stuck in one end...

      If an M10 tap has been used, then presumably metal has been removed, so complete reforming won't be possible. You might get it so that the mating screw goes in, but how much strength will there be remaining? Going deeper as Mike suggests is a good way around this. Is the spindle hardened? If so, maybe that's why the previous attempt with an M10 tap failed.

      All of the gear, no idea...


      • #4
        Maybe drill the hole a bit deeper and run the right size tap in. You should be able to source a longer bolt easily which will then have some new threads to hold into.
        My concern would be getting the tap to start following the original thread, rather then the metric one, and buggering it up worse. Drilling out only the first few threads (the M12 ones) should hep avoid this. You'd also want to be sure that there were enough good threads left once you got past the removed/destroyed/no good anyway ones. But replace rather than repair does sound safer.
        "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979


        • #5
          to be honest, unless it's a high zoot BMX axle, if it has threaded holes at the ends it's from a really cheap bike. I'd go with the two Ms and drill tap deeper. Weird to find a SAE thread on a bike though, bike fasteners have been metric for decades even in the mother country.


          • #6
            I think you would have more luck with getting a standard tap in the hole than a roll tap.


            • #7
              I don't know what a 'roll tap' is, but if it is the same as a form tap it is not likely to work. Since form taps work by moving metal, there will be problems where metal has been cut away by the conventional tap. Additionally, the form tap will shear previously cut threads in spots and you'll have no idea what strength remains.
              Southwest Utah


              • #8
                I originally suggested helicoils but that idea was rejected.
                Who and why was that idea rejected?
                Home, down in the valley behind the Red Angus
                Bad Decisions Make Good Stories​

                Location: British Columbia


                • #9
                  Originally posted by mattthemuppet View Post
                  to be honest, unless it's a high zoot BMX axle, if it has threaded holes at the ends it's from a really cheap bike. I'd go with the two Ms and drill tap deeper. Weird to find a SAE thread on a bike though, bike fasteners have been metric for decades even in the mother country.
                  Since the OP neglected to say whether it's a motorcycle or pedal bike and front axle or rear axle, "bike" is pretty generic and no help whatsoever.

                  I don't know how many threads I've seen where the OP gives a minimum amount of information and then several threads later after getting a plethora of "solutions" finally gives some detail of what the part is/does and that renders a lot of the reply posts moot.
                  Location: Saskatoon, Saskatchewan, Canada


                  • #10
                    I'm waving in agreement with Arcane here.

                    3/4" and inch size thread suggests an older vintage MOTORcycle of some sort. I don't know of any older bicycles that used anything near to that size for an axle. And only newer metric bicycles use tubular thinner wall through axles on some applications. And the spline you mentioned and the single seating screw at the one end suggests it slides into a front fork and is then clamped by the fork legs. Seen that on one of my past motorcycles. But a metric one, not imperial sized. Not sure why there's a spline but odder things have been seen before.

                    I don't know about your 4140 assessment but certainly any and all bicycle and motorcycle axles are heat treated. They would simply not stand up to much punishment otherwise. Using a form threading tap on metal which has a spring temper and will fight the forming should be.... er... "interesting" to say the least.

                    I'd be very careful if you choose to bore out and press in or "glue" in a new insert. The plug you add would never be part of the load bearing part of the axle so keep it small and don't go overly deep. I would suggest that it not even be as long as the clamped and supported area of the fork leg it fits. And if it's even too close to the end of the support and depending on what the motorcycle is used for and how hard it is ridden there's still a chance that it could shear at the end of the bored out hole for your plug if it's too close to the end of the support from the fork leg clamping. If your buddy is worried about a Helicoil removing metal I can't see him liking this plug idea.

                    Next up for least damaging would be your original idea of a Helicoil. Not sure why your buddy dismissed this idea. For any solution that requires removing metal it's the least intrusive. Perhaps get back to him and convince him.

                    Next would be to clean up the threads with the proper 3/8-24 tap. And since an M10 isn't far off the target the first few threads SHOULD aim the 3/8 tap in the right direction since the original 3/8 threads would have done the same for the M10 tap or bolt that did the damage. And at that point and assuming you manage to get a bottoming tap to run clear to the end of the hole tell your buddy to get a bolt that is longer and cut and file until it just barely seats the axle like 1 turn before the bolt bottoms. That should ensure that it doesn't strip out the obviously damaged threads.

                    And last of all... How deep do the messed up M10 threads extend? If it fits well enough how about just sigh and use an M10 bolt? The damage is done already.

                    And of course I jumped to all the same conclusions that Arcane suggested and I waved at in agreement.... But I THINK I'm pretty safe on the MOTORcycle connection and the idea of it being off a Brit or Yankee made vintage bike.
                    Chilliwack BC, Canada


                    • #11
                      Helicoil or Keensert Done.


                      • #12
                        good point chaps I didn't catch the diameter, that's definitely not bicycle wheel axle, though it might be a bicycle bottom bracket axle. Who knows?


                        • #13
                          Ah, sorry guys. The thought it might need differentiating from a motor bike part never occurred to me.
                          mattthemuppet has hit the nail on the head that it's a fancy vintage BMX axle - and I'm probably wrong in using the term "axle" here as it's probably technically a shaft (I'm not the one with the bike fetish case you couldn't tell!) that goes between the pedal crank arms. Hence the reason for not simply replacing it and also my preference to not go for the nuclear option if possible and keep the original thread.

                          Deeper may definitely be an option if, as has been suggested, it isn't hardened. Although it's in a state currently, the friend wanting fixed is good at the bodywork and painting side of it.
                          It's also worth mentioning that these threads don't seem to be either load or torque bearing, the splines take the torque and the CSK head screws (and CSK 'washer' I'll have to make) really act to pull the crank arms on and retain them.

                          Thanks for the info on the roll tap (aka form tap). I'll grab a normal cut tap and see if I can either get it started or drill/bore out the first few damaged ones (thanks for that suggestion) and then run it in. Options if that should fail depend on if/how hard it is but I suspect if hardened much, it might be easier to plug it with an unhardened cylinder and tap that rather than try to tap it directly. I don't have the kit (or ability) to anneal it let alone re-harden and temper it.


                          • #14
                            I would say it's like this, a BMX bottom bracket crank shaft. It will be hardened and is chrome moly so it will be hard to drill or tap. These as Mattthemuppet said have been metric for decades so is the owner maybe wrong in there assessment of what it should be? A manufacture name or a search on the net might get you a replacement, this one is $11.00 online in stock. Spline count does vary as well as length, so be sure to get all the info if replacement is shopped for.

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                            Mr fixit for the family


                            • #15
                              These are apparently by Profile and do see to be north of a hundred bucks and use a UNF thread. Bolts available (in the US - so hideous postage, import and duty costs) confirm it is 3/8-24
                              It's useful to know that it's actually 'hardened' as opposed to 'hard to get a hand tap in'.
                              ​​​​​​​I don't have any problems machining the EN19T and EN24T stock I have and it can't be fully hardened or it would be liable to hopefully with some carbide I ought to at least be able to take out a few threads to expose the undamaged ones. I can but try.