The other day, I replaced the bottom bracket on a customer's commuter MTB with a square taper shaft
※Since it's 29 inches (700C), it's a bit ambiguous whether it should be classified as a cross bike, but I went with MTB since the crankset is MTB-type
I ordered a Shimano BB-UN55.
And this left adapter—
It's metal!
Shimano's square taper bottom brackets have
part numbers starting with BB-UN, and
the current models are three types: BB-UN55, 26, and 100.
The old XTRC had the 90-series designation BB-UN90/91,
and below that was the 70-series
BB-UN70/71/72/73, which are all discontinued,
then came the 50-series
BB-UN50/51/52/53/54/55,
of which 55 is the current production and top-tier model.
Below that was BB-UN40, which is also discontinued,
then the 20-series with BB-UN25/26
of which 26 is current production,
and the lowest grade is BB-UN100, which is also current production.
Long ago there was a BB-UN10,
but that uses a completely different installation tool than current models.
Also, to be precise, there were other bottom brackets besides UN-numbered ones
like BB-LP28 and BB-TY30.
So with the UN-series bottom brackets, as you go down in grade,
the left adapter becomes plastic,
and that causes play and noise in the crankset.
Back in the day, we'd often replace it during maintenance
or swap it for a metal left adapter from a higher-end model right from the start.
With current production, only the 55 has a metal left adapter; the 26 and 100 are plastic.
The 40-series and
20-series definitely have
plastic left adapters,
but the long-running 50-series model
BB-UN52 has both metal and plastic left adapters
despite having the same part number.
↑2000 model Tiagra
↑2001 model Tiagra
So the later model switched to metal... or so you'd think,
but BB-UN53 and 54 also have
versions with plastic left adapters.
What's going on here?
That being the case, I thought the BB-UN55 would also
have a plastic left adapter,
but it turned out to be metal, so I was surprised.
After checking, it looks like
the BB-UN55 is 100% metal left adapter across the board.
I used to stock up on the metal left adapters from the BB-UN72,
but when I checked just now, all the 90-series and 70-series parts
except for the crank fixing bolt on the 71 have been discontinued.
※Since it's 29 inches (700C), it's a bit ambiguous whether it should be classified as a cross bike, but I went with MTB since the crankset is MTB-type
I ordered a Shimano BB-UN55.
And this left adapter—
It's metal!
Shimano's square taper bottom brackets have
part numbers starting with BB-UN, and
the current models are three types: BB-UN55, 26, and 100.
The old XTRC had the 90-series designation BB-UN90/91,
and below that was the 70-series
BB-UN70/71/72/73, which are all discontinued,
then came the 50-series
BB-UN50/51/52/53/54/55,
of which 55 is the current production and top-tier model.
Below that was BB-UN40, which is also discontinued,
then the 20-series with BB-UN25/26
of which 26 is current production,
and the lowest grade is BB-UN100, which is also current production.
Long ago there was a BB-UN10,
but that uses a completely different installation tool than current models.
Also, to be precise, there were other bottom brackets besides UN-numbered ones
like BB-LP28 and BB-TY30.
So with the UN-series bottom brackets, as you go down in grade,
the left adapter becomes plastic,
and that causes play and noise in the crankset.
Back in the day, we'd often replace it during maintenance
or swap it for a metal left adapter from a higher-end model right from the start.
With current production, only the 55 has a metal left adapter; the 26 and 100 are plastic.
The 40-series and
20-series definitely have
plastic left adapters,
but the long-running 50-series model
BB-UN52 has both metal and plastic left adapters
despite having the same part number.
↑2000 model Tiagra
↑2001 model Tiagra
So the later model switched to metal... or so you'd think,
but BB-UN53 and 54 also have
versions with plastic left adapters.
What's going on here?
That being the case, I thought the BB-UN55 would also
have a plastic left adapter,
but it turned out to be metal, so I was surprised.
After checking, it looks like
the BB-UN55 is 100% metal left adapter across the board.
I used to stock up on the metal left adapters from the BB-UN72,
but when I checked just now, all the 90-series and 70-series parts
except for the crank fixing bolt on the 71 have been discontinued.