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Found spec sheet, but what BB length do I need for my crank?
#1
Looking to rebuild the entire drive train on an old frame. To start, I'm looking at the Shimano Tourney A070 double crank set. But I can't make head or tails of the spec sheet I've found for a compatible BB? Shimano's recommendation is the BB-UN26, for which there are at least 5 different lengths (from what I can tell from some additional googling).

In the "Axle length / BB spec" section, there are 8 categories, all applicable to 68mm (BC1.37) or 70mm(M36) shells:

HOLLOWTECH II construction
109.5 mm
118.5 mm
113 mm
121mm
MM107
MM110
LL113

"MM110" looks to be the one applicable to me (LL113 if I were going for the A073 triple). What are those? What do they mean? Does this tell me what the actual length of the darn thing needs to be?

The old spindle was a 3T with a double crank and is ~126/127mm (w/o the threaded bosses). This threw me for a loop, since Sheldon Brown lists 3T at 123mm...
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#2
68 x 110 ; if staying with Shimano, get a UN55.

Note, you also need to purchase bolts to hold the cranks to the BB. Some brands include bolts, Shimano usually does not.
Nigel
  Reply
#3
Thanks (again) Nigel!

Anyone have any insight on why a 110mm BB is categorized as MM110 (and, similarly, why a 113mm is categorized as LL113)? This is especially perplexing considering the otherwise intuitive convention for 109.5, 118.5, and 121 mm sizes.

And what's the difference between 113mm and LL113, anyway?
  Reply
#4
unknowledgable person doing data entry.
Nigel
  Reply
#5
(11-11-2013, 01:26 PM)nfmisso Wrote:  unknowledgable person doing data entry.

LOL!

Well that would explain a lot.
  Reply


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