I’m stumped. The BB shell is ever so slightly smaller than BB cups. I was planning to shuffle things and install thread together BB in the BB30 shell but I guess that’s out now :S
by slowcaptain
I’m stumped. The BB shell is ever so slightly smaller than BB cups. I was planning to shuffle things and install thread together BB in the BB30 shell but I guess that’s out now :S
by slowcaptain
18 Comments
It is pressed into place, and it should stay in place when you bike, that’s why it’s tight!
It’s a press fit bottom bracket, so the shell has to be smaller otherwise the cups would just fall out
FYI: a delta of 0.1mm or ±0.05mm (0.002″) is within the error of using uncalibrated Amazon calipers. Inner diameter measurements are tricky to measure accurately with calipers anyway. If in doubt, pay a bike shop a couple bucks to break it instead of yourself.
I think that between the fact that the tool, and method, for measuring can cause this difference, and a rubber mallet not being the tool of choice to press in a BB, these should be fine and you should get a LBS to fit it for you.
Take it to a bike shop, pay them to do it right with a real BB press. It won’t be that expensive.
If they are mean to fit together then sand carefully around new bb. May sound extreme but sometimes any coat of paint etc. is thick enough to knock off the tolerance… Sounds like even a oil film is too much in this case. Heat and cold may work also but those are indeed extreme measures and could go wrong very easily – most likely.
That’s how they stay in. It compresses ever so slightly. Even the thread together ones.
Still need to press these in and not go at it with a hammer. Super even pressure is the key.
Sand them evenly. Check every so often as you sanding. You should be able to take off enough material for a very snug press fit.
It’s a press fit, also known as an interference fit. The friction caused by the slight difference in diameters holds everything in place. You need to install them with a press tool so that they go in straight, not just so it’s easier, but so you don’t damage the surfaces. If they are damaged, then they might not hold in place properly.
To measure an ID correctly you need a bore gauge. The inside distance measurement on a caliper will always say the ID is smaller than it actually is. How much smaller is a ratio of the tool thickness to bore diameter.
Using a mallet might work with a headset cup on an alloy mtb but it sure as fuck is a horrible idea for a pressfit bb. You need a diy (threaded rod, washers, bolts) or real pressfit tool or just to take it to a shop.
Get LBS to ream and face the BB on your frame. Tools too expensive to justify it for a one off job.
This is just normal machining, it’s not wrong.
Even though CNC machines are used many companies give a tolerance.
0.10-0.50 is about right for something like a bike part, the tips in the CNC machine wear and you cannot guarantee that every single one is exactly the same, hence the tolerance and they are done in big batches, so say the company that machined them made 500 of these at a time, they may have checked 1 in 6 or something like that.
It will fit believe me and you don’t need another part.
As other people have suggested get someone with the correct tools to press it in for you and get the thread in, I have seen pulleys that you can’t hammer or push in the centre ring but with a press it glides in, it’s just so it’s a super tight fit.
Don’t sand them, that defeats the purpose of the fitting all together.
Source- The family business is a machining company and I’ve made odd parts there for my work in construction.
Inside thread outside thread measurements?
Tried cooling the bb? Throw it in freezer for an hour and it may shrink enough to fit it in.
It’s an interferance fit. It’s normal and expected.
If you don’t know what an interferance fit is : [https://www.mec-engineering-spreadsheets.com/documentation-area/fit-tolerances-and-applications/](https://www.mec-engineering-spreadsheets.com/documentation-area/fit-tolerances-and-applications/)
If you are looking for the fit tolerance of your BB standard : [https://www.hambini.com/support/technical-specifications/bottom-bracket-drawings/](https://www.hambini.com/support/technical-specifications/bottom-bracket-drawings/)
You can’t reliably measure hundredths of a millimetre with a caliper.
Please take it to the shop and have them install it for you easily instead of DIY coz you use mallet which is not an ideal to use and it will likely damage the cup or the bb or even both. I read all the comments and they are right about the size. It’s tight af but it’s correct and it’s design not to fall off…it’s normal. So basically you need a proper press fit tool if you really want to do it yourself but my suggestion is bring it to your nearest bike shop because they know what they are doing and they do it all the time no problem instead you only do once a year maybe. I mean they can get it done less than 5 mins without going to reddit or facebook and ask question.
Don’t force it.. Get an accurate measurement, then go for it…
[https://www.youtube.com/watch?v=zJJuG3We_6w](https://www.youtube.com/watch?v=zJJuG3We_6w)
Idk how many times people need to tell you. **YOU STILL NEED A BEARING PRESS FOR THREAD TOGETHER BOTTOMBRACKETS.**