I have removed the bottom bracket from my bike as it was very wobbly. I am trying to teach myself about these things and I am making progress however the bottom bracket looks weird. Granted I have not seen a ton of them, but aside from the current BB saying NFC07P There are no other markings on it.

Here is a link to the same bike as mine.


Here are the bike specs. Silverback Stride HD 2018 29'R Crank set is Shimano 175 FC-TY301, 42/34/24T

I have looked at a compatability guide for shimano products and it appears that I can slot in a Shimano UN-300 BB to fit my crank set.

I am unsure about wether it should be a 63 or 73mm BB. Being an MTB is it safe to assume that a 73mm would be the correct option?enter image description here


1 Answer 1


Don't make the assumption, spend 2 minutes and do the measurements.

The width of the Bottom Bracket(BB)-shell on your bike should be either 68mm or 73mm. You then measure the length of your spindle on the BB you removed, which could be something like 122mm for instance. Using these two measurements you select the proper matching BB.

We have then assumed that a BSA(aka English, a common standard) type of BB is what your shell is made for*. In addition you can try to match the chainline which is designated by some letters (D-NL/D-EL) but this is less important than the other two critical numbers and more tricky to measure since you have removed the crankset already.

*) Even if your crankset can use the BBUN300, this is no guarantee that your bike's shell can fit the BBUN300. You can do some measurements and check thread direction to see if you in fact have a BSA/English standard shell. From park tool:

"The common threaded bottom bracket uses cups or adapters with the thread specification of 1.37″ x 24 threads per inch (approximately 34.8mm diameter). The most commonly used term for this standard is “English” threading, sometimes abbreviated as ENG. The non-drive side (left side) on the English standard is a right hand threading, tightening clockwise. The drive side (right side of the bike) uses a left hand thread, tightening counterclockwise. The shell inside diameter will be approximately 33.7mm."

EDIT: If you at some point install a BB for the first time, make sure not to cross thread it. A usefull tip is to rotate it slowly by hand the wrong way and when you feel a sudden soft click/clunk you start turning it the correct way for tigthening. Check several times from the non-drive side that things look properly aligned and no excessive force should be needed until the final tightening.

Good luck!

  • 1
    Thanks for the help, I have measured and it's 73mm x 122.5 or 123. Appreciate the help and advice. Feb 17 at 22:44

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