So if I have a frame, how do I know what size bottom bracket to get?

I assume if I measure the inner diameter of the bottom bracket shell (is that what it's called.. the hollow cylinder on the bike?), I should just buy a bottom bracket that is the same diameter... or should it be slightly less to allow it to fit?

And for width, do I just measure the width of the shell, or do I add/subtract a couple millimeters to determine the proper BB width?

Finally, what is the most common BB width and diameter; like is there a standard that most bikes seem to fit?

Scratch that...

The threading most likely will be English, but depending on the frame it might be italian. What kind of frame are we talking about?

As far as spindle length, that depends on what cranks you are running.

...um, no, the width of the bb shell will be 68mm (if its english)
or 70mm (if its italian).

****, right.

Why did I think it was diameter?
Forget that.

ok, what i was referring to as diameter, i think you guys are referring to as width.

in that case, what about the other dimension... from the left side of the shell to the right side of the shell (i guess you could call it "length"). how do i determine the correct length of BB to get?

also, in terms of english vs. italian threads, this applies to the threads on which the cranks will screwn onto, or is the BB also threaded into the bike frame?

haven't chosen a frame yet, but just educating myself so i know what parts to purchase, and what to stay away from (e.g. if there's some kind of rare standard that i should avoid)

thanks again

bb's can be a can of worms. get the frame & crank, then worry about it.

Yeah, the frame and cranks will determine what size and kind of BB you get.

Bottom Brackets come sized 68x110 or 70x110 (those are examples, there are many other variations)
The smaller # is the BB shell width, the larger # is the spindle length.

As Travsi corrected me, 68mm is standard for pretty much everything, but some Italian bikes will be 70mm. Then there are 71mm, 73mm and 76mm shells, but don't worry about those.


Engligh vs. Italian threading has to do with the BB threading into the frame.

As far as cranks connecting to the BB, you have a few options here as well, but that's another story...

there's also 73mm, more of a mountain thing.

and the rear hub chainline...

ok, makes sense now. so how do you determine the proper length BB for your frame? should it sit flush with the frame, or stick out the ends a little? What should I measure on the frame to determine the proper BB length?

Spindle length?

That is based on cranks and rear hub.

This thread is an amazing example of what it means to put the cart before the horse.

+1. The shell width is determined by your frame. It affects nothing else, and there are no substitutes. If you have a 68mm shell, english threaded, you will NEED a 68mm BB with english threading. Simple as that.

Spindle length determines how wide the spindle is, from tip to tip. The cranks press onto the spindle, and the BB comes with the bolts/nuts that hold the cranks on, so these threadings aren't something you have to worry about. What you do have to worry about is the TAPER of the cranks and the CHAINLINE that the cranks will give you.

TAPER is determined by the Crank/BB manufacturer. Shimanos, Suginos, and Stronglights use a JIS taper, Campy and Miche use ISO taper. Phil Wood makes BB's for both tapers.

CHAINLINE is determined by your rear hub. Most track hubs have between a 42 and 44mm chainline. You will want a BB that pushes your crank out far enough (or pulls your crank in far enough) to get a similar, if not the exact same, chainline.

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