Bicycle Mechanics - Bottom Bracket Question

I just replaced my BB in my Trek 4300. I ordered a 73 mm and for the most part the install went fine. (my first time) However, i have like 1 mm sticking out on each side. The cups are as tight as I can get them. I looked at my "old" (3 month old) BB that i took out and it has 68 stamped on it, yet it measures, 73 mm.

What gives? Also will my BB be ok with a little bit sticking out?

As long as you put the BB in first and tighten it down as far as you can you should be fine. Just put the cup in all the way. I had a 4300 and don't remember the BB size. From the pictures everything looks good.

What do the markings, BC 1.37 68 Mean? Can i tell spindle length by this? I want to ebay this BB but can't determine the spindle length for sure. When i eyeball measure it, it is around 129 mm. I don't want to call it a 129mm and have someone buy it and then tell me i was wrong. :(

Are you sure there is only 1mm sticking out of either side? Did you tighten it a great deal to get only 1mm to stick out?

I believe BC refers to English threading, 1.37 in the BB diameter and 68 is the width. It'll say the spindle length on the box if it doesn't have it on the actual BB.

If the spindle width is the same, then it probably doesn't matter if a little sticks out of both sides, as long as the insides of the cranks don't rub against the protruding BB.

Also I'd recommend getting a digital caliper for measuring such things as spindle width. You can find them for 15 or 20 dollars. I ordered mine from Harbor Freight Tools. It is probably from China, but it works well.

I believe BC refers to English threading, 1.37 in the BB diameter and 68 is the width. It'll say the spindle length on the box if it doesn't have it on the actual BB.

If the spindle width is the same, then it probably doesn't matter if a little sticks out of both sides, as long as the insides of the cranks don't rub against the protruding BB.


Yep BC is English 1.37 in (34.8mm) and 68 is the frame's BB shell width in mm.

What BB do you have? From the info on the box, its make/model, etc. you should be able to verify the spindle length. To be honest, I've never measured mine, just knew what length I needed to get based on the BB I was replacing. But 129 mm seems pretty long unless its an ES-51, is your chainline still the same w/ the new BB?

Dave

Yep BC is English 1.37 in (34.8mm) and 68 is the frame's BB shell width in mm.

What BB do you have? From the info on the box, its make/model, etc. you should be able to verify the spindle length. To be honest, I've never measured mine, just knew what length I needed to get based on the BB I was replacing. But 129 mm seems pretty long unless its an ES-51, is your chainline still the same w/ the new BB?

Dave

It's a long story. Long story, short, my LBS put too wide of a spindle in. The put the 129mm in. I replaced it with the spindle length,spec'd by the crank manuf. (122) It's wierd that they put in a 68 when my shell measures 73. I put a 73 in and you can see that it sticks out a little bit but not, 5 mm!. I think the flange they used must have compensated for the smaller BB.

Either way, i replaced my own BB and I am happy. The bike shifts a TON better and I did better than my LBS did. In fact my bike shifts better than it ever has, and also better than my other identical bike. Spindle length is obviously important to avoid chain rub. I have adjusted all of the chain rub in all combos out of my front Der. I thought that would never happen!!!

IN the last week i have replaced my first set of cables and my first BB. Next week: new shifters/brake levers. This is pretty cool!! :D