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Discussion Starter · #1 · (Edited)
At 52 I have built a lot of frames up, but recently ran into something I have never encountered when installing a bottom bracket into a new frame. The frame is a Salsa Fargo, and as noted its brand new. I checked the frame over really well when I received it and the BB threads looked great. After prepping the frame with Boeshield and letting it dry-out a day, I wiped out the threads and coated the shell and BB threads with some anti-seize and went to install my fairly new shimano UN72 BB.

No dice on the drive side, it just would not take. I have three different Un72's so I tried each of them with the same results. I had simply resigned myself to take the frame in to have the threads chased at an lbs, but decided to try another non-shimano Race face BB I had in the parts bin and It threaded right in! I ran this cup in and out a few times and was thinking awesome, now hopefully I can get the shimano to thread now that I have installed a cup and cleaned the threads a little.

No luck, not happening with the exact same results as before. Unfortunately the race face is not the right spindle length and its ISIS to boot and my cranks are square taper. In the end I robbed my SS of a nice new FSA Ultimax ti spindled BB and like the Race Face BB it threaded right in.

Oh well, at least my Fargo is benefiting from a nice new lightweight ti BB. Now the really wierd thing is that the UN72 that would not thread into the Fargo threads right into my SS! Very odd situation. The only thing I can see that may be different is that the drive side cup on the shimano appears to have less taper prior to the thread starting point when compared to the other two brands. Anyone else ever run into anything like this?
 

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I am currently running a UN72 in my Fargo with no problems. I also ran a UN92(?)(old XTR) in it as well with no problems. I have no suggestions as to how this happened on your Fargo.
 

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Discussion Starter · #3 ·
Well, its good to know that its not a Fargo specific issue, and considering I have three different UN72's to choose from, I know its not the BB itself. Must just be the luck of the draw on my frame, and probably having the threads chased would have resolved the problem. Although the FSA Ultimax BB thats currently in the bike is light, I really like my UN72's best as they are pretty much bullet-proof and not too heavy considering the durability factor.Thanks for the info.
 

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It might not be a general Fargo problem, but could be a thread alignment problem specific to your bike.

The UN72 BB cups fit fairly tightly over the cartridge and must therefore thread in to a frame where the BB threads on either side are on exactly on the same axis. Any mis-alignment in the threads will cause the cups to bind.

Test for this as follows, thread each side alone to see if the cup and BB threads are OK. If so, thread the cartridge in with the attached cup and eyeball it to see if the other end is centered in the shell. If yes, next try installing the opposite cup most of the way, and see if the cartridge will thread in from the other side to meet it.

The above method will help identify off axis BB threads, which can be corrected by running a bicycle specific BB tap set (the kind where both taps are connected to a common shaft) through, but only if they're only slightly off. If the mis-alignment is significant, the only answer is a new frame.

On the off chance that there's only a tiny amount of mis-alignment, sometimes you can get the BB installed by greasing the overlaping area and bringing both sides in progressively allowing the system to split the difference. If you try this, be very sensitive to increasing torque as the threads cam slightly, and if it continues to ramp up, be ready to give up before jamming things in so tight they can't come out.

Good luck
 

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Finally! Something to do with my profession!

It doesn't have to do with thread alignment or any of that stuff. It's simply the BB tube on the bike specifically, or Shimano specifically. I thread pipes on a CNC all day and can tell you that a tiny, tiny, tiny chip in a threading insert can be the whole difference. Even though a guage will fit, the actual parts might not fit on together correctly. OTOH, it can be the threads in the BB tube too. I don't know the tolerances in bikes, but when we do pipes to drill into the ground we're working with .003"- The thickness of a human hair. Down to .0002"- a hair divided by 15! It doesn't seem possible, but it is. My conclusion, take it to the shop and let them try a Shimano in there to see if a different one than yours will work. If not, just run RF. Ttyl, Fahn
 

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LarryFahn said:
Finally! Something to do with my profession!

It doesn't have to do with thread alignment or any of that stuff. It's simply the BB tube on the bike specifically, or Shimano specifically. I thread pipes on a CNC all day and can tell you that a tiny, tiny, tiny chip in a threading insert can be the whole difference. Even though a guage will fit, the actual parts might not fit on together correctly. OTOH, it can be the threads in the BB tube too. I don't know the tolerances in bikes, but when we do pipes to drill into the ground we're working with .003"- The thickness of a human hair. Down to .0002"- a hair divided by 15! It doesn't seem possible, but it is. My conclusion, take it to the shop and let them try a Shimano in there to see if a different one than yours will work. If not, just run RF. Ttyl, Fahn
Great theory, and it could be a tolerance issue, except that bike BB threads aren't an especially close tolerance. As for it being either one specifically, reread the OP.

He tried three different Shimano BBs in the frame, no dice so either they're all NG or it might be the frame, except that the frame readily accepted other BBs and so would seem OK.

The problem is limited to the specific combination of BB and frame, and not either one alone. While CNC threads may be your profession, bikes are mine. The need to have two threads with near perfect axial alignment is a unique application which rarely comes up outside of bicycle bottom brackets. Threads may be of perfect form, and dimension but unless they share a common axis a single part will not thread into both simultaneously.

BTW- in the bygone days when BBs were tapped and faced after the bike was finished, the problem was eliminated by tapping both sides at the same time with taps constrained to a common axis. Also the BB facing process used interlocking inserts which couldn't be installed when the threads were mis-aligned creating a built in quality control check.

This is an old and common problem familiar to most experienced bike mechanics, but may not be the OPs exact problem,which is why in my last post I offered ways to test for it. If the OP is unsure of the methodology and knows a dealer with a Campy or equivalent tool kit, have their mechanic try installing the guides (pictured here) of the bottom bracket facing tool. If both sides fit but not at the same time, you'll know for sure.
 

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Discussion Starter · #7 ·
Hey guys, I originally was installing just the drive side alone. It would not take. I have a steady hand, and was eyeing the spindle on the other side to make sure it was centered. Then I also installed the non-drive side cup by itself, not totally threaded in but about 7/8 of the way in. and then slipped the BB back in from the drive side using the other side to help index and align it. This made no difference and the issue was the same regardless of whether the other cup was in or not. The bottom line is that the bottom bracket wasn't binding in any way. I even removed the cable guide screw to make sure it wasn't hitting on the body of the BB, but that was not the issue.

The threads and the shell appear to be true and straight, and as long as I use the non-shimano bottom brackets, they both threaded in very easily and there was absolutely no sign of binding or mis-alignment. As I mentioned the only thing that differentiates the shimano from the other two bottom bracket cups (Race Face and FSA) is that the latter two both have a longer tapered section on the leading edge of the cup leading up to the threads of the cup, and the shimano cups have a very short taper before the threads. The non-shimano BB's thread right in with no problem whatsoever. It may simply be that the thread profile of the Race Face and the FSA are slightly tapered at the beginning part of the threads, and the Shimano threads are not. I am pretty sure that if I had the BB threads chased the shimano would also thread right in as well. As it stands I avoid bike shops whenever possible, and for now the FSA BB has found a new home and the UN72 is in my SS.

Thanks for the feedback guys good information and thought process. If I do have the LBS chase the threads and it works I will provide info back into the thread.

Merry Christmas guys!

Jeff
 
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