I hope somebody here can help. I'm trying to replace the bottom bracket on a second hand bike with a new Shimano UN26, but having trouble getting the old one out. I don't know what make or model it is - only that it's ~68mm x ~122mm.

I can take out the non-driveside cup fine, but not the driveside (which is the cartridge body). The tools I have simply won't fit the slightly strange looking cup. Here's what the driveside end looks like:
https://imgur.com/s2x3hhF
So it has 20 splines, and 6 odd looking external splines.
The text around it reads "BC 1.37" x 24 TPI-LH". Might be RC, not BC. As far as I can tell this is just the tap size.
Does anybody know what bottom bracket this is?

I have two bottom bracket tools.
https://imgur.com/YBGSk7L
However, they both seem too big to fit in - not sure if this is the size/shape of the teeth, or the diameter of the circular bit at the bottom of the teeth. Between the two of them, they fit the splines in the Shimano UN26 pretty well, so theoretically one of them should work with this.

I could shell out another £15 and see if the Park Tools BBT-22 fits it. Or do I need a tool that uses the 6 external notches? Or a different 20-toothed tool?

Ta muchly,
Rufus

Is the BB drive side a plastic material? If so then the splined tool tends to be a tight fit. I've had to use a tap hammer to seat them before. Andy.

I would've guessed a Shimano BBT-22 or BBT-32 would likely fit the 20 internal splines, but those are also the same tools that would fit your new UN26 bottom bracket. The 6 external splines look like the Park Tool BBT-4 bottom bracket tool for Campagnolo might fit (but that's obviously not a Campagnolo bottom bracket). It shouldn't matter whether you use the internal or external splines, so long as you've got a tool that'll grab and turn.

What was the tool interface on the other side?

Seat the spline tool with hammer, that fails use channel locks, since you already have nds side out hit it with some WD40 from that side before removal. I'd also brush out the spline before setting tool to give best chance of getting a good bite with tool

I'll ask the obvious. Are you aware the drive side threading is left-handed? You turn the tool clockwise to remove the cup or cartridge.

Yes, it's a plastic material on drive side. Non-drive side is just 20-tooth, but one of my tools fit that fine.

And yup, I know it's left-threaded, I cannot even get a tool in to turn it the wrong way currently.

Thanks for the responses so far. I'll try and seat with a hammer tomorrow.

It didn't want to go anywhere when hammered, so I'll take it to a shop and ask them to try the 6-notch tool on it.

One problem with most all the BB cup tools is when you have to apply massive force (to loosen the cup) and also keeping the tool engaged with the cup (as in not slipping off). I prefer to use a splined tool, compared to the outer ring/notched tool) because you can use a long lever arm (to the point of clamping the tool in a bench vice and turning the frame). To prevent the splines from disengaging during this effort some form of a clamp is needed to tighten the splined tool onto/into the cup. The simplest is to use the axle bolt and washers/spacers to bolt the splined tool against the cup.

Some Euro BBs have the plastic BB cups with both splines and outer notches. SKS, Thun amongst others, Andy.

It turns out an *** 20-tooth tool did it fine. Thanks for the inputs.

Seriously? The word F . A . G got filtered.