Bottom Bracket Size

[pacman924]

I have a Giant AC2 and i'm going to get a new bottom bracket but i cant figure out waht size to get can anyone help me out?
Thanks

[-dustin]

all of my bbs have the size stamped on them.

[speedbump]

You need the BB shell width (usually 68mm or 73mm) and the spindle length. Measure the shell width on your frame and the spindle length on your old BB (assuming your not changing cranks).:)

[binary visions]

The Giant AC's take a 68mm bottom bracket. Spindle length is up to you. If you're running a chainguide and/or big tires, I'd recommend a 118.

So, in case you don't know BB sizing, when you order it the sizes will be something like:

68x113
68x118
73x113
...etc.

First number refers to the actual width of the BB shell, the second number is the length of the spindle. 113 is fine if you're not running a chainguide or big (~2.7) tires.

[Tame Ape]

How important is it to get the threading length correct? What if I order to short, or correspondingly, to long? Also, how will I know what the proper length for the threading should be?

I am hopefully going to be moving from a Cortina 4130 HT to a Heckler...

[binary visions]

Originally posted by Tame Ape
How important is it to get the threading length correct? What if I order to short, or correspondingly, to long? Also, how will I know what the proper length for the threading should be?

I am hopefully going to be moving from a Cortina 4130 HT to a Heckler...

The threading length is important to get right.. you can use a set of calipers or a ruler to see how wide your BB is. Very easy - it's either 68 or 73 (few of them are 100mm but it's not common). To tell you the truth, I've never tried to put a BB in that was the wrong size. Too short, you'd risk not having proper overlap between the non drive side cup and the rest of the BB. Too long probably won't even fit in there because the shape of the BB would probably cause the main section and the cup to bind before it was bottomed out on the frame.

You can buy BB's that will fit either - RaceFace makes a bunch.

[Tame Ape]

Is it measurable with a caliper? I was not sure if it was refering to how far the threads were threaded in the BB shell or the type of threading...

Thanks for the quick reply though!

[binary visions]

Nope, it's just the width of the BB shell itself. For a non-metric minded person 73mm is just shy of 3 inches (about 2.9") - if you think about the width of your BB, that makes sense.

Anyhow, like I said a bunch of manufacturers make BB's that will fit both widths. If you're concerned about compatibility when you switch bikes, just buy one of those. Probably your best bet is to pick up a 118 spindle if you're planning on running a chainguide or wide tires.

Incidentally if you're looking for a Shimano ES-70 BB (their Octalink spline), I have about 4 of 'em in varying spindle lengths and shell sizes. PM me if you want one.