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Bottom Bracket: Replacing UN72 with UN55. HELP!!!

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Bottom Bracket: Replacing UN72 with UN55. HELP!!!

Old 04-07-20, 01:09 PM
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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.
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Old 04-07-20, 01:29 PM
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Originally Posted by Papa Tom
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.
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

Last edited by noobinsf; 04-07-20 at 01:36 PM.
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Old 04-07-20, 01:41 PM
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Would the threads be different?
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Old 04-07-20, 01:48 PM
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Originally Posted by Papa Tom
Would the threads be different?
No, but the spindle/cartridge is longer to compensate for the wider 73mm bottom bracket shell for which it is intended (vs the 68mm shell that you have). Your cups are bottoming out on the cartridge contact points.
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Old 04-07-20, 01:51 PM
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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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Old 04-07-20, 01:51 PM
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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.
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Old 04-07-20, 02:00 PM
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"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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Old 04-07-20, 02:25 PM
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Originally Posted by noobinsf
"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.
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??
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Old 04-07-20, 02:34 PM
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Originally Posted by Papa Tom
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??
Gulp, I hope I was right!

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?
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Old 04-07-20, 02:52 PM
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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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Old 04-07-20, 03:04 PM
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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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Old 04-07-20, 03:04 PM
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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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Old 04-07-20, 03:07 PM
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Originally Posted by noobinsf
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
Ironically, I was just experiencing the opposite problem. Need 73mm, but had about six Shimano sealed BBs ALL 68mm wide. I visited the LBS and couldn't find the 73 X 127.5 needed for my frame & crank. Could NOT find a Shimano in this dimension anywhere online. My old sealed Specialized/White Ind. Ti 68X122.5 spindle BB worked, but with only a hair's width of crank/stay clearance on NDS. Went with an off-brand that ended up working just fine. Considered Phil Wood, but this was a "parts bin" build, so that was definitely out of the question.
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Old 04-07-20, 05:47 PM
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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.
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Old 04-07-20, 05:47 PM
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Originally Posted by dedhed
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.
Pick up a small (2" diameter approximately), circular wire brush attachment at the local hardware store. In a drill, it will do wonders for old BB threads! It will clean them out like never before. Doing this -usually- obviates the need for chasing threads with a BB tap, which the LBS will charge decent money to have done. Also, remove any BB cable guide screws when doing this and installing the new BB. They usually get in the way and make threading in the "fixed cup" (DS, reverse thread) side difficult.

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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Old 04-07-20, 05:55 PM
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Originally Posted by LV2TNDM
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!
LOL, "I Have"
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Old 04-07-20, 06:06 PM
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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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Old 04-07-20, 06:21 PM
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Originally Posted by Retro Grouch
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.
Hmmm, would the old bracket have come OUT if that screw was in the way?

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?
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Old 04-07-20, 07:00 PM
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Originally Posted by Papa Tom
Am I still missing something?
Well, if you measure the bottom bracket shell, it will remove all doubt. If 68, you’re golden with the new purchase. If 73, the issue will not be solved with the new purchase, and it’s prob not too late to cancel the sale. Just sayin’...
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Old 04-07-20, 08:50 PM
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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.
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Old 04-07-20, 10:47 PM
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Originally Posted by blakcloud
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.
You know, I saw that, and I also saw that the 95 does not have the bb shell width listed. It's possible one could have bought a 95 bike in 96, and that it was a transition year.
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Old 04-08-20, 12:17 AM
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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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Old 04-08-20, 05:57 AM
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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.
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Old 04-08-20, 06:27 AM
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It may have been less expensive and less frustrating if you would have taken it to a LBS.
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Old 04-08-20, 06:32 AM
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Originally Posted by Tandem Tom
It may have been less expensive and less frustrating if you would have taken it to a LBS.
You're tellin' me! But I am on quarantine and won't go into a bike shop. Plus, I'm bored as hell and have changed three or four of these before, so I figured it'd be something to do and not such a pain in the butt.
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