Bottom Bracket: Replacing UN72 with UN55. HELP!!!
Some time ago, I got the bug to order a spare bottom bracket for my 1996 GT Outpost so I could replace the current one (getting old) in my spare time. Well, now that I have nothing BUT spare time, I put on my "shop" clothes this morning and set out to do it.
The problem is, the folks here on this forum insisted that a Shimano UN55 would be the replacement for the impossible-to-find UN72, so I bought the 55. So far, it ain't workin'. I can't screw either side of the bracket all the way into the frame. On each side, there is about 1/2" of protrusion. Is there a trick to making this work, or is my brand new UN55 a doorstop now? And is there a better replacement for the UN72? The only details printed on the UN72 are ""LL113." The 55 reads "73 BC1.37 X 24." I'm full of grease, and I am lost. |
Originally Posted by Papa Tom
(Post 21405659)
Some time ago, I got the bug to order a spare bottom bracket for my 1996 GT Outpost so I could replace the current one (getting old) in my spare time. Well, now that I have nothing BUT spare time, I put on my "shop" clothes this morning and set out to do it.
The problem is, the folks here on this forum insisted that a Shimano UN55 would be the replacement for the impossible-to-find UN72, so I bought the 55. So far, it ain't workin'. I can't screw either side of the bracket all the way into the frame. On each side, there is about 1/2" of protrusion. Is there a trick to making this work, or is my brand new UN55 a doorstop now? And is there a better replacement for the UN72? The only details printed on the UN72 are ""LL113." The 55 reads "73 BC1.37 X 24." I'm full of grease, and I am lost. https://www.jensonusa.com/Shimano-UN55-Bottom-Bracket |
Would the threads be different?
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Originally Posted by Papa Tom
(Post 21405718)
Would the threads be different?
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The threads should be the same if you ordered an English threaded UN55. If by some chance you got an Italian threaded one, nothing is going to match.
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LL is probably the length of your spindle end to end. What you probably need to be concerned with it the width of your frames BB shell. And that might be some of your issue, but not likely to be the 1/2 inch on either side.
Is it possible that the un72 wasn't original to the bike? Perhaps the shell got mucked up from a previous substitution. |
"1.37 x 24" represents British threading, so that should be correct. Looking into it a bit, it looks like a 73mm bottom bracket shell is not uncommon on MTBs, but I suspect that standard took hold after your mid-90's MTB was produced. Another example -- mid-90's MTBs are likely to have 130mm rear dropout spacing, whereas 135mm is the current MTB standard.
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Originally Posted by noobinsf
(Post 21405751)
"1.37 x 24" represents British threading, so that should be correct. Looking into it a bit, it looks like a 73mm bottom bracket shell is not uncommon on MTBs, but I suspect that standard took hold after your mid-90's MTB was produced. Another example -- mid-90's MTBs are likely to have 130mm rear dropout spacing, whereas 135mm is the current MTB standard.
ANYBODY WANNA BUY A UN55 BOTTOM BRACKET?? |
Originally Posted by Papa Tom
(Post 21405786)
Noob: Followed your advice and ordered the 68 X 113. the guys at Jensen didn't give me too much confidence that it would fix the problem, but I will give it a try. Thanks for your help.
ANYBODY WANNA BUY A UN55 BOTTOM BRACKET?? Seriously, though, it's hard to think of what else it might be, and this is the first thing I would try. Judging by your bike list, I presume you've had this bike since new, and that it hasn't sustained any damage to the BB shell? Also, do you happen to still have the old UN72, and can you compare the length of the two cartridges? |
I'll also say due to the very fine thread you need to clean out the old ones really well, slowly reverse thread the new cup until you feel the threads drop, start going the correct direction, and screw in by hand until you're sure it hasn't cross threaded. They are very easy to cross thread, especially on old abused bikes.
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If you feel that the cups won't screw into the bottom bracket threads, you might try testing the tread pattern with just the cup (no spindle & bearing). It's a little easier to get started and feel if it's cross threaded.
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Also, I realize we've overlooked an obvious step -- measuring your bottom bracket shell width. Your two possible readings are 5mm apart, so a tape measure or ruler should do, no need for a micrometer.
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Originally Posted by noobinsf
(Post 21405697)
You have the 73mm version of the UN55, but you need the 68mm UN55. Here is a link to a listing for either (use the drop down list to see all available sizes). You could sell the 73mm, which I think would be in use in a vintage Raleigh, among other bikes (with a wider shell).
https://www.jensonusa.com/Shimano-UN55-Bottom-Bracket |
As much as I enjoy bicycle mechanics, I am lazy about doing things like measuring the bottom bracket width, etc. I just want to tell somebody which bike I own and have them sell me the correct part. Once it gets into measuring millimeters and I have to pull out my dollar store reading glasses, the fun is over.
The UN55 I tried to use as a replacement has a slightly wider axle than the UN72. Noobie and two techs at Jensen suggested that I need the 68mm - rather than the 73mm version of the UN55 to be able to thread it all the way in. We'll see, but now I'm sorry I dirtied the new UN55 I opened today. If anybody needs one, let me know. |
Originally Posted by dedhed
(Post 21405829)
I'll also say due to the very fine thread you need to clean out the old ones really well, slowly reverse thread the new cup until you feel the threads drop, start going the correct direction, and screw in by hand until you're sure it hasn't cross threaded. They are very easy to cross thread, especially on old abused bikes.
PS OP, are you related to Tom Papa? (Obviously not, I realize.) His "Out in America" segment on "Live from Here" kills me. Love him! |
Originally Posted by LV2TNDM
(Post 21406141)
PS OP, are you related to Tom Papa? (Obviously not, I realize.) His "Out in America" segment on "Live from Here" kills me. Love him!
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One other simple possibility that hasn't been mentioned yet: Do you happen to have a screw in cable guide under the BB shell? If so, take out that screw and test fit your bottom bracket again.
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Originally Posted by Retro Grouch
(Post 21406173)
One other simple possibility that hasn't been mentioned yet: Do you happen to have a screw in cable guide under the BB shell? If so, take out that screw and test fit your bottom bracket again.
Also, I have cleaned the inside of the frame THOROUGHLY with a wire brush and some simple green. You could eat your lunch off the thing if you wanted to. Still can't thread the damned BB all the way through. The fact that so many of you are still offering advice on how to make the current UN55 fit is not giving me confidence that the purchase I made today is the solution to my problem! Three people have now told me I have the wrong variation on the UN55 and that the one I have on order should fit perfectly. Am I still missing something? |
Originally Posted by Papa Tom
(Post 21406199)
Am I still missing something?
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That bike has a 73 mm shell. Old mountain bike and I found the specs listed here.
Just measure it to confirm. Two possible solutions have already been mentioned, cross threading and bottom bracket cable guide. Again someone else mentioned using the old bottom bracket cups to clean up the threads, then try the new bottom bracket. |
Originally Posted by blakcloud
(Post 21406501)
That bike has a 73 mm shell. Old mountain bike and I found the specs listed here.
Just measure it to confirm. Two possible solutions have already been mentioned, cross threading and bottom bracket cable guide. Again someone else mentioned using the old bottom bracket cups to clean up the threads, then try the new bottom bracket. |
Old BB came out fine? Comparing the old BB cups with the new BB cups, do the cups on both have similar amount of threads? If yes, you should be able to thread the new cups all the way in. Maybe just use some force, grease up the threads nicely, and work it in.
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The old cartridge was a UN72 and it had the number 73 on it. I'd assume that means it's a 73mm.
When I read all the specs to the FIRST Jensen rep and told him was noobie said, his response was "That sounds about right." Not too convincing. When I had to call Jensen back because their website wouldn't complete my purchase, I asked a second rep and he gave me even less confidence. Shouldn't these guys know a little more about this? Once I was assured the bracket was returnable, I ordered TWO, just so I'd have the "right" one next time I need to replace it. If I get stuck with two - PLUS the one that's not fitting - I'm giving up cycling forever and taking an Uber on the bike trails. Also, YES, I have thoroughly cleaned the threads with a wire brush and I have backed off and re-tightened many, many , many times. Today I will try removing the cable-guide screw. |
It may have been less expensive and less frustrating if you would have taken it to a LBS.
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Originally Posted by Tandem Tom
(Post 21406896)
It may have been less expensive and less frustrating if you would have taken it to a LBS.
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