isis BB sizing?

[brussel]

OK so in another thread there is a ritchey crank being sold that uses an isis BB.

 

I am riding a Giant anthem, how do I determine what size BB to buy? they come in a bunch of sizes. Obviously I need to match the shell size for the relevant bb shelll on my anthem...what is the other measurement...the axle length?

 

help....

 

[Johan Bornman]

Yes, the other size is the axle length. To not mess with your chainline the new one should ideally be the same. It is measured over the axle and will be something like 118 mm or 113mm. If the BB is off the bike you'll see the size written on it somewhere. If there is a 68 on there it tells you the width of the BB shell and indirectly whether it is English or Italian. 68 is an English thread BB (Italian is 70mm) and the 1.36 refers to the thread pitch. If you've established that you have the right "language" and right width, you've matched everything you need to match.

Johan Bornman2009-03-09 10:44:58

[brussel]

OK fair enough so I have checked and I need a 68mm. But how do I measure what spindle length I need. My current crank is a hollowtech and has a built in axle.

 

Of course if I simply use the chain rings I will not even have to buy a BB

 

[Johan Bornman]

OK fair enough so I have checked and I need a 68mm. But how do I measure what spindle length I need. My current crank is a hollowtech and has a built in axle.

Of course if I simply use the chain rings I will not even have to buy a BB

 

It is purely guesswork and if you don't have the luxury of experimenting with various ones like shops do, you can do some informed guesswork. BBs are supplied as 113mm, 115mm, 118mm and some other special sizes. Most common is 113/115, with 118 on some tandems and other bikes with chainstay clearance issues.

 

Therefore yours is either 113 or 115. Look at your current setup. If there are lots of spacers in there, go for a 115. If you only have one, or no spacers between BB bearing cup and crank, go for 113mm.