tigerteeuwen

Hello all!

so i am building a bike for winter commuting, the frame i am using is a Norco bigfoot.
during the strip down phase on the bike i noticed the bottom bracket has quite a bit of play.

I did some research online and it seems the bottom bracket is a 68mm english thread.
I also looked up my crank which is a Biopace exage m300 300LX, and based off my spindel length is 121mm.

Now based off of this i can't really find a square taper to match up to this so i am thinking, switch to a more modern crankset, i am looking at a alivio fc-m410 which will work for a 7 speed rear end.

I just wanted to make sure of one thing with you experts though, do i match the bottom bracket exactly the same as my prior one measurement wise? because i found a shinamo es25 that comes in a 68mmx 121mm spindel english thread.

Please let me know guys as i want to get this on order.

thanks so much

IthaDan 

Can you or can't you find a bb with the same spindle length? You say both. Figure you have about +/- 3mm on spindle lengths, so somewhere in the general area of 121 should be fine.

shoota 

The spindle length you need depends on whether you are running a triple or double up front. The 68mm English threading is just for the cups that lock the spindle in place. The es25 works with octalink V2 cranksets (which the Alivio is). But I'm not sure why the crankset is listed as 7/8s, I guess it has something to do with the tooth spacing. But anyway it sounds like you are on the right track.

wphamilton

I don't know your specifics, but that sounds like the ubiquitous Shimano UN55 bottom bracket.

tigerteeuwen

sorry i wrote that a bit confusing, I couldn't find a bottom bracket in square taper in a 68mm x 121 spindle length, so i was contemplating if i should just update to a non Biopace crankset.

The Shinamo UN55 seems to come in a 68mmx118mm or a 68mmx122.5mm
would these still be compatible with the chain line? probably not worth the money though to spend on a bottom bracket such an old crankset.

CACycling

Is the crankset in good shape? If it is, there is no reason to replace. I would suggest measuring the existing BB spindle to verify the actual length. Also, check to make sure it is not asymetrical. If 121 is correct and it is symetrical, the 122.5 BB would move your crank 3/100" further out which won't be a problem.

Bill Kapaun

Is the current BB a cup & cone type with loose spindle?
IF so, look on the spindle for a "code"
It'll be something like 3-P, 3-S or 3-something.
That will help determine WHICH spindle you have and the correct cartridge to get to keep your rings where the FDER can operate them.

Newer cranks tend to use shorter spindles.
IF you get a new crank, use the BB specified for it.

https://techdocs.shimano.com/media/te...9830667768.PDF

onespeedbiker

The answer to your question is no. The bottom bracket spindle length is specific to the crankset; usually you'll find it mentioned with the crank such as this Amazon ad https://www.amazon.com/Shimano-FC-M41.../dp/B00288RSES. The Shimano doc linked is a little difficult to understand as it covers both Octalink and Square Taper versions and includes a few other bottom Bracket options. Basically all you should need to know is if you choose the Square Taper crank you will need a 68x UN26 BB 113mm (LL); if your front derailleur is attached to the bottom bracket (E-Type) than you need a UN26(E). The K designation is for a narrower chainline (47.5) which is bike specific and shouldn't concern you, as the 50mm standard (no "K") works with almost all bikes.

tigerteeuwen

Thank you everyone for answering my question. Ended up going with a octalink bottom bracket and crank-set as well as the specific tools to install them. Can't wait to keep working on this rebuild.

DX-MAN

OP's question is already solved, but I feel the need to address this.

Alivio is listed as 7/8 because it will work with 7- and 8-speed systems; shouldn't be that tough to figure out, since 5, 6, 7, and 8-speed systems all use the 3/32 chain.

shoota 

So if you put on 9s everything else with that crankset what would happen specifically?

IthaDan 

Nothing.

....Maaaaaaaybe you'd be able to shift the chain between the chainrings in a no man's land of sorts, but you can adjust 99% of that out with a little diligence.

shoota 

Exactly.