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I have a 2016 Merida Big Seven 100 27,5" XC MTB which I bought from second hand market. It came with a 68mm cartridge style BB. However I found out that this bike has originally 73mm BB. What could go wrong by using 5mm shorter BB? Btw, when I measure the bottom bracket shell of the bike it is 71mm.

Below is the screenshot from Merida's catalog. It doesn't state BB shell width though. My source for 73mm is 99spokes.com. The link is further down below.

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https://99spokes.com/en/bikes/merida/2016/big.seven-100

This is the BB I currently have: https://www.bikecart.co/products/kenli-bottom-bracket

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    A photo of the non-drive side cup would be helpful.
    – mattnz
    Commented Feb 5 at 20:24

2 Answers 2

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If you mismatch this in either direction (73 in a 68, 68 in a 73), even if everything else works you'll be offsetting your cranks by 2.5mm from where they were "supposed" to be.

Many cartridge BBs have a lip on the outer edge of the NDS cup. It's intended to sit right outside the edge of the shell, maybe 1mm away from it, and I believe it's there to provide extra material at the tool interface when the cup is plastic or aluminum. If you put a 68 in a 73 shell, the lip will bottom out against the shell and the cup won't be able to go in far enough to support the BB. Even if it didn't (a steel NDS cup with no lip for example), it might also bottom out in the frame threads.

enter image description here

All you need is a ruler to know what shell type you have. It's just the face-to-face width of the shell.

If you were in a resource limited situation and couldn't get the right thing, many of them can have their bearings replaced by doing some crafty things to get the unit disassembled. You might find the replacement bearings are more attainable.

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  • BBUN300 is lipless on the NDS, I assumed the one installed "originally" also was lipless as it has done the job (hopefully for 1000s of kms)
    – WornChain
    Commented Feb 5 at 16:29
  • My BB has no lip on the NDS. I removed it because it was creaking. It was sitting inside the edge of the shell. I have chain rubbing problem on the biggest ring and smaller cassette combination. I think the reason is small BB width.
    – Ender
    Commented Feb 5 at 17:48
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The bike might have been 73mm originally, then someone at some point shaved off 2mm when (re-)facing it. If a 68mm BB did the job and fits, then I would be inclined to just replace it with a similar 68mm BB.

Using a too short BB could cause the BB to be improperly supported at the non-drive-side(NDS) and not give the proper preload after installation if threads don't run deep enough. The crankarm/bearings on the NDS will also be offsett 2-3mm towards the DS compared to the ideal scenario. I doubt this has major implications if the support/preload is ok.

These are just my thoughts, I have no experience with installing too short BBs.

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