HELP! Broken threads, can they be saved?

[snowjim]

Hello,

Unsure where this thread should be placed but I'll try here.

I bought a nicer exercise bike second-hand and the first step is of course cleaning. A larger screw (32mm) in the front turned out to be quite stubborn to unscrew.


The image shows a similar screw but this one is on the back near the seat.

When I finally got it out, I inspected the threads and can see that they are quite worn down! I should have perhaps suspected this considering how the head of the screw looked.


http://i.imgur.com/1hGiZip.jpg


By feeling around in the grease, I was able to fish out a couple of larger metal pieces that are likely to have been parts of the threads.



The large screw is removed during service of the machine and has another screw inside that presses on the brass cylinder you see in the hole. This, in turn, presses on the handlebar post so it doesn't fall down. That is, the screw needs to be able to be mounted and removed.

I know you can buy a small tool to rethread, but I suspect it's anything but easy to get it straight and right. Furthermore, the hole will become larger which means the special screw won't fit anymore?

Of course, I've contacted the seller (a gym) but I'm afraid they'll refer to the existing condition even though it stated mint condition in the ad.

Any suggestions for a neat solution?

 

[cpalm]

Presumably, the thread has been damaged by tightening the "screw" at an angle. With some luck and patience, you might manage to get the screw to catch the original thread and then use a bit of force to "clean" the thread. Hopefully, what's left of the thread will still be sufficient for it to serve its purpose.

 

[Pagno]

You drill out the threads and insert a new thread like "Helicoil" which you fasten with Loctite! It is a relatively easy procedure if you are into cars or similar hobbies, but if you don't have the knowledge it might be a bit difficult the first time! The important thing is not to drill too far in, to drill with the right drill diameter to fit the new thread, unfortunately, most people don’t have 32mm drills in stock at home! Feel free to search for helicoil if you want to know more, take care and good luck.

 

[cpalm]

Yes, it's precisely the dimension that makes it extra tricky, otherwise, threaded insert would be a good alternative.

 

[Pagno]

Alternatively, if it's possible to insert a smaller helicoil without drilling out the old threads and then use a thread cutter to reduce the threads on the "screw" to a smaller dimension.

 

[Stefan1972]

If the screw is around 30mm, the helicoil kit might end up costing more than the bike itself...................probably even more than what the bike cost when new.......
Try cleaning the threads with a thread file to start with. If you're lucky, you might get it to align correctly again and go back into the right thread.
Otherwise, it's probably easier to do a bit of DIY and come up with another solution.

 

[snowjim]

Unfortunately, it doesn't seem easy to find a helicoil in size 32mm, although I only did a quick search on Google. I also don't think I have the tools to drill a 32mm hole in stainless steel. Feels like I would need to take it in to get it done. I can imagine it will be quite expensive.

 

[snowjim]

Presumably, the thread has been stripped by screwing the "screw" at an angle. With some luck and patience, you might manage to get the screw to engage with the original thread and then, with a bit of force, "clean" the thread. Hopefully, what's left of the thread will still be sufficient for it to fulfill its function.

Yes, I have tried this but when you think it’s straight and start to tighten, it unfortunately slips out. Of course, I can screw it at an angle again, but in the long run, I guess it won't hold.

 

[snowjim]

Alternatively, if it's possible to insert a smaller helicoil without drilling out the old threads and then use a tap and die set to reduce the threads on the "screw" to a smaller dimension.

The downside of this is that it becomes more difficult to replace the screw in the future. But sure, the screws probably hold well despite being mounted and dismounted a couple of times a year. However, it sounds like expensive tools? Don't you also need to be able to clamp the products straight in some vise?

 

[Jumos]

Isn't it possible to drill a completely new hole above/below for the same function, but with a different dimension and entirely new components?

 

[snowjim]

If the screw is around 30mm, the helicoil set is likely to cost more than the bicycle itself.................probably even more than what the bicycle cost new....
Try cleaning up the threads with a thread file to start with. If you're lucky, you might get it to go straight again and engage the right threads.
Otherwise, it's probably easier to DIY a bit and come up with another solution.

Aha, thread file? It looks like the inner threads are intact and nice, it's the outer ones that are really bad. I found this at Biltema:


http://www.biltema.se/sv/Verktyg/Skarverktyg/Gangverktygssats/Gangfilar-2000016881/

Can something like this actually solve the problem? I understand that new threads can't be generated, especially when some material is gone, but if you can get the screw in so it grips well in the inner threads, that should be enough. I also saw there's something called a thread gauge?

Of course, it will be around 100 kronor but it's worth it considering the bike cost 5000 used (from a gym).

If you think this might help me, I'm willing to try it. There's also a tap, which I've tried before, but I suspect it's just as difficult to get it right as getting the existing screw in correctly.

 

[Norrhyttan]

What function does the screw serve? If it's not too heavily loaded, it's probably enough to clean the thread with a thread tap (which you probably don't have, but can likely borrow from the local truck workshop in exchange for some pastries) and be careful when handling the screw further.
If a thread insert is the only solution, once again the truck workshop or another mechanical workshop is my recommendation. Buying your own tools to fix a single thread is just absurd.

 

[richardtenggren]

That gang file is solely for internal threads, right? You can use a bottoming tap to clear the internal thread, along with a die for the external thread, can't you?

 

[snowjim]

I spoke with the seller (gym) and they insist they have never touched this screw, even though it has tool marks. They, of course, believe it is something we did, which it clearly is not; I know how to screw in and out screws, after all, I frequently work with cars that have similar dimensions on screws/bolts.

One option is to weld the screw in place, which would mean you can't replace the upper plastic cover, but maybe that's something you can live with.

The best solution would be if we could fix the threads so we could at least get the screw into the undamaged part, but that doesn't seem straightforward. At least not if you want to keep costs down.

A thread file sounds like an option, after all, it's only 100 SEK, but if that doesn't help, it's not very fun.

Of course, I could struggle more with it and force the screw in even if it goes in crooked; it would likely damage the hole, but it still doesn't feel right, and it's visibly crooked from the outside.

 

[snowjim]

That thread file is only for male threads, right? Can't you use a bottom tap to clean the female thread, and a die for the male thread?

It looks like the screw itself is intact and fine, but the problem is in the cycle itself. So what you're saying is that a bottom tap is what's needed? Can you get it to follow the intact inner threads while also cleaning up the outer edge? Are there any for 32 mm?