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Discussion Starter · #1 ·
Here are two things I don't understand about these XT BBs - I'd be grateful if somebody would put me right:

1. the top two are 68/107 and the bottom one is a 68/110. The 110 has a 3mm longer spindle, but can anyone explain why the 3mm difference is all on the non-drive side, and all three are identical on the drive side? i.e., why a 68/107 gives exactly the same chain line as a 68/110?

2. the top one has an alloy adjuster weighing 11g, the middle one has a heavy adjuster that looks like a cheap worn-out item and feels like a steel weight. OK, 23g isn't the end of the world, but when I received it I wondered if I'd been ripped off. But I have seen a picture of a new UN73 with an adjuster that looked quite like this one, right down to the flattened threads, so does anyone know whether it is a proper XT part?.

Many thanks
 

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These are obviously older square tape BB's. If I remember correctly, you could get these & other Shimano models in many widths, i.e. 107, 110, 113 etc. The width needed was , many times determined by the frame manufacture & frame design. The drive side could not vary too much, or else the proper chainline would not be achieved.
Not sure what you mean by 'adjuster', if it was the loos cup that screwed into the frame to hold the BB, Shimano would actually use an alloy, steel or some type of plastic composite(cheap) depending on the model. being 10 grams is less than half an ounce, I wouldn't be concerned, the steel actually, IMO, held up better.

JimPz
 

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Not a real answer to your question, but if you need to scooch it over just a bit to the drive side there are spacer/shim washers that you can put between the frame and the drive side cup to give you a milimeter or two or three.
 

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Some older cranks require the drive-side arms to have more offset than the non-drive. Some BBs came with tapers to fit these cranks. Phil Wood still sells BBs like this if you need them. Phil sells "symmetrical" or "R+3" (which is offset 3mm on the drive-side or right side).
 

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retrobike refugee
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1. So you didn't understand FluffyChicken's response! I didn't either, and I'll wager neither did he. I don't know why Shimano chose to do it that way.

2. The UN73 was introduced some time after square-taper cranks had gone from the upper ranks of Shimano's range, and seemed to be an attempt to cut costs on Shimano's part, hence the one-piece shell and cup on the drive side. Shimano's cheaper bottom brackets had long used a steel or plastic cup on the left: steel for the 107mm model and nylon for all the longer models. Nylon is cheaper (and lighter) than steel and is adequately strong, but requires an external strengthening flange. Steel is used for the one case (107mm) where an external flange might compromise clearance between the shell and the crank.

If I had to guess why Shimano chose to taper the thread of the steel cup, I'd say it's to allow for a slight misalignment between the two parts on installation. Nylon or aluminium would yield slightly. That is the correct part.
 

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All the UN7X BB's I've had have come with the aluminum non drive side cup. The UN5X BB's (I believe) come with the steel cup, and the lowest end ones have come with the plastic cup.

They're all interchangeable, and if you picked it up used, it's possible that someone damamged or swapped a cup in an earlier life.

My preference is actually the plastic cup, no corrosion issues to worry about, and they seem to be the most creak proof, as there's no metal on metal creaking. You have to be diligent with your BB removal tool and make sure the splines are all engaged, but that's good wrenching anyway (and it's easy to strip the aluminum splines as well).

Plum
 

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retrobike refugee
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All the UN7X BB's I've had have come with the aluminum non drive side cup.
That may well be so; but if it's so, you've never had a UN73.

The UN5X BB's (I believe) come with the steel cup, and the lowest end ones have come with the plastic cup.
At some point during the lifetime of the UN52, Shimano switched the steel cup for a nylon one in all lengths except 107mm. I believe that's remained the status quo for all subsequent iterations.
 

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Discussion Starter · #8 ·
yosnappyj said:
1. So you didn't understand FluffyChicken's response! I didn't either, and I'll wager neither did he. I don't know why Shimano chose to do it that way.
I hadn't actually seen Ben's response, but as you mention it I'll reproduce it here. I think you're a little unkind to him. I find I can't always understand his explanations, but I don't assume that that means that he can't. Anyway, lots of people are better at understanding things than they are at explaining them - for example most college lecturers.:madman:

He says "It's an in fill length for 107/113 the chain line of 47.5 and 50mm respectively. It came after those lengths had been used for M900/UN90, probably as feedback from manufacturers, (from catalogue info). Like many I always assumed it was to give a middle length, but not so, it's to give a greater none drive side clearance if needed to balance the gaps out. something like that, it also allows a greater chain-line alignment with drive side spacers and not ramming the non-drive side into the stays.

Internally I think it's a symmetrical bottom bracket whereas the 107/113 are internally asymmetrical. (not externally though.) Would need to check that though. If memory serves it harks back to the transfer from cup and cone lengths and shimano cartridge equivalents."

The chain line depends on the cranks and Shimano were making bottom brackets for use with other makes of cranks, not just their own. A smaller increment than 3mm would be useful in finding 47.5mm with some cranks.

Incidentally Sheldon Brown says that a 47.5mm chainline is best for mtb/triple chainset with a 28.6 seat tube. He says that a 50mm chainline is only suitable for 31.8 and 34.9 seat tubes, as they set the derailleur cage further outboard. I must confess that I didn't know that until I read his page, and it surprised me, but if he says it it must be true.
 
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