Bicycles Stack Exchange is a question and answer site for people who build and repair bicycles, people who train cycling, or commute on bicycles. It only takes a minute to sign up.
Sign up to join this community
I ordered an Italian threaded bottom bracket, model number SM-BBR60. The box has two check boxes, one for 68mm and one for 70mm. The 70 mm is checked.
My bike has a 68mm bottom bracket shell. Everywhere I look shows this model as only one size. But the checkboxes?
Does this bottom bracket come in two sizes? And is 70mm useless to me?
The width for externally bearing threaded BB's is not very important. What is important is the threading. If you have an italian BB it has the same right handed threading on both sides. You can in effect make an italian BB with external bearing BB's by buying two english BB's and only using only the right hand threaded external bearings.
In this case the difference between 68 and 70 is the difference in the two threadings. I have the italian version of this BB on my italian threaded bike and there is enough slop in the plastic housing in between the bearings that I suspect it is exactly same part. My guess is that the only difference btw 68 and 70 is the 68 comes with BSA threads( one right, one left) and the 70 comes with two right handed.
Since the crank you would use with this BB is almost certainly designed for a 68mm BSA BB, you will have to tweak the spacers a bit to get the optimal chainline.
Having said all that, unless you are working on a vintage steel bike made by an italian company, it's somewhat unlikely that you have an italian threaded BB.
I did a web search with the model number, and the first hit is to Shimano product page that says that the bottom bracket comes in 68mm and 70mm widths. That, and the fact that your bottom bracket is marked as 70mm, suggests that the bottom bracket indeed comes in two sizes. If you are still unsure, you can use a ruler to measure the actual BB.
The 70mm model is useless to you. If it was sold to you as 68mm model, you should be able to return it. Even if you mistakenly bought the wrong model, you should still be able to replace it.
