A friend of mine just bought a bike and brought it to me to check out. He had ridden it twic, the first time it was great and he loved it, the second time was great but it seemed to have developed a "thump in the front wheel". I immediately noticed that the non-drive side cup looked like it was halfway out. Looking closer at the bottom bracket spindle on the non-drive side was off-center vertically and horizontally. Looking at the drive side is was slightly off center. Spinning the cranks with my hand it seemed to spin smoothly.

I removed the NDS crank and then the cup with no issue. The drive side cup took several attempts over the course of two days to remove. It is a carbon frame with an aluminum bottom bracket and because of that I was worried about over torquing and damaging the frame. I laid the bike on it's drive side and sprayed WD-40 from the NDS to an let it sit between attempts to loosen the drive side.

Once it came loose, as expected, the threads are damaged on the bike. It is a Shimano BB-UN25 1.37x68 (English) cartridge bottom bracket and the threads are undamaged on either side. This model bottom bracket has a plastic cup on the non-thread side which seems insane to me and since it has already worked itself loose, I am replacing it with a bottom bracket that I have laying around.

I don't have a bottom bracket thread tap laying around and as I understand it, if this can be repaired there is one chance to get this right. Even if I did have the tool, having never done such a repair and the fact that it is not my bike I wouldn't try repairing this myself.

Do any of you have any experience with this issue? Is this repairable?

Difficult to say (especially without a photo of the "damaged" BB shell threads).

I'd take it to an LBS (or two) and see what they say.

+1 with FMB, with out pictures or looking at it is super hard to tell you if it will work fine or not.

My best guess is that probably the 1st 3 or 4 theads are not good but if that thing is ok deeper than that you wont have a problem if you can get somebody to tap that again.

Second option, you can do your own tool but you will need an old cup and then file like 4 slots around. We did one like 20 years ago using a shimano dura ace cup because the steel was super sharp and super hard. Just an idea ok? but is your call.

Hope you find somebody with the tools.

The key as to whether it cn be repaired is the exact nature of the damage, and how much material is left. In many cases there's distorted thread with maybe some material missing and it can be repaired. However, often especially when there's cross threading, or corrosion damage, too much material is massing and there's not enough to retap. In those cases it is possible to tap it out to an Italian (36x24) thread, and this has been the standard save for expensive frames with non-salvageable BB threads.

Either way, it's not just a matter of owning the tools. Some skill, experience and judgement is called for to ensure that the appropriate method is used.

Was there a relationship between the thump in the front wheel and the bottom bracket observation?

As it seems more than one aspect is out of wack, recommend not attempting any repair on the bottom bracket, but instead holding off until you can hook up with someone who has the proper tools (threading tools, facing tools, etc., basically the Campy toolkit or equivalent) and knows what he is doing with them. Without a picture...I would say at best your friend is about an inch away from needing to go shopping for a frame again so best to put that last inch in the hands of someone with tools and experience.

Suggest also looking around for any solutions that avoid the damaged threads....I remember someone mentioning Velo Orange guys website for repair kits that sleeve in or press in to a damaged one.

I will post pictures after I get home tonight. First two bike shops I called wont touch it.

If you post your city of residence, perhaps someone can refer you to a knowledgeable shop close to home.

There are some BBs that bypass the threads, but may not satisfy the carbon frame bike buyer..

to revert to square taper cranks.


back to blind guesses..
maybe the aluminum BB insert is separated from the CF .. partially.
by force used to remove the siezed part

the BB was installed dry , I suspect..

may have been a false bargain, and you have a bunch of components,
then need to buy a new frame..

2nd hand CF bikes seem to be problems somewhat often on these posts.

[OK, no problem = no reason to post, Re what is not happening]

Velo Orange sells a threadless bottom bracket for this very purpose.

https://store.velo-orange.com/index.p...-brackets.html

+1

There is another manafacturer that produces similar BB's but I can't remember who offhand. They were also pretty cheap from what I remember. I'd go with VO.

+1 on the VO BB. No machining is required and unlike tapping out the BB to Italian thread, you can remove it and have another go with the existing threads at a later time if you choose.

I'd stay away from the makeshift tool idea; if the starting threads are damaged it may not be possible to get it started straight and you might find yourself in a worse situation than you started with.

the plastic lock rings on the UN26 are SUPPOSED to be installed dry.

You may be able to pickup the correct threads using a BSC drive side cup (I would try this with a cartridge). Take the cup and carefull strat threading the cup while keeping it centered and straight. When it binds, tap it with a rubber mallet and try screwing it in some more; you may need to utilize a tool, but be vary careful. As long as the cup is going in centered and straight you'll be alright. If the cup slips off center, it is picking up the old crossed threads. Back it off until the cup it straight again, tap it with a mallet and try threading it again. The beginning of this process is critical you keep the cup centered and straight if it's going to work. If all you end up doing is chewing up the threads stop and take to your LBS. Also the reason why the non-drive lock ring unscrewed is usually because it was no torqued enough. The plastic lock rings are torqued as tight of as the alloy cups, but many are afraid to do so and it may cause the non-drive side to back out..

My apologies for the delay in getting these pictures up, it's been Monday all week at work.

The threads are crossed but using my fingernail as a gauge it seems to me that they should be repairable.



I suspect that everything was installed correctly but that the plastic cup was not properly torqued and came loose while riding. Then the bottom bracket slipped and perhaps the torque on the cartridge from pedaling caused the drive side cup to slip, cutting across the threads which lead to the "thump" my friend experienced while riding. I have checked the tire, tube, rim, hub and spokes and there is nothing that I have found that accounts for the thump.

The wheel is slightly out of true, the nuts are to tight on the bearings, and it all the bearings need grease - both hubs and headset. I plan to replace all the bearings/add grease this weekend.

a carbon fiber frame with a UN25 BB? wtf. thats like the CHEAPEST BB put on low end hybrids and such. I usually replace those with UN56's.

but yeah, I'd think running a thread tool through that and facing the BB should be workable.

What he said. But it depends how bad the threads are.

The threads look salvageable, but it's not a job for an amateur. Your best bet is to visit a knowledgeable shop and let a pro do the job right. Ask a lot of questions, and if you don't feel confident, try somewhere else.

If you post where you live, someone should be able to steer you to a reliable mechanic.