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Replacing BB
I apologise if this has been asked before.

I would like to change the BB on my wife's Mountain bike. At the moment it has cup and caged bearings with a square tapered spindle.
Is it possible to replace it with a cartridge type one.

Yes it is, consider shimano un 54 or such. IMO let dealer do it as special tools are needed........Should be correct width.
Never Give Up!!!
Yep. You measure the width of your old axle, accurately, in millimeters. That is the second critical size #. It'll be something like 122mm, but it could be a little under 110mm. The first # is the width of your frames BB area, which, unless exotic will be 68mm.
So my example would be 68 x 122.5mm. These are available from funky off-brands to the common Shimano variants. Shimano models will be UN-## then the size. So a cheapie would be a UN 26 68 x 122.5, better, with aluminium instead of plastic parts would be UN 55 68 x 122.5. I prefer all steel Tange BBs. $15-$30-ish
You'll need to decide if a local bike shop should do it for you. You'll need common tools + some bike specific, a crank puller to get the old crank (pedal) arms off. $15-ish
You'll need a BB tool to install the new bb, $20-ish also.
An LBS would charge about an hour labor. If you have the tools it's pretty easy & fun.
You will need a pin tool to a standard or homemade tool to remove the fixed cup - it's not simply finding something to fit, as the cup is on extremely tight. As you may never need the BB tools again one way to approach it would be to take apart the BB except for the fixed cup, have the shop remove that, and then either you or the shop installs the cartridge bearing. It's very important to torque the crank arms sufficiently when reinstalling.
Thanks for your kind replies, I already have the tools to get the old one off as I grease the bearings quite regularly.

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