Hi
Quick query. I’m looking to replace my front rotor (currently a Shimano 180mm, part number SM-RT54-M). It’s a centrelock fitting, with external notches on the lock ring. When I look for a replacement nearly all appear to have internal notched lockrings. Are the discs the same and it’s just the lockring that varies? Or is the external notch lockring a different fitting than the internal, if that makes sense? Any help appreciated!

The externally notched ring, uses a BB tool to undo it, whereas the internally notched ring uses a cassette tool. Why the difference? Because if you run 15 or 20mm axles, there isn’t enough space to get a tool into the “Hole” in the middle of the internal ring type, so you need to use the external notch type!

So it is simply the tool required that is different, the rotor itself is the same whether it comes with an internal or external lock ring? I have the relevant BB spanner to remove/refasten…

Yup, rotor is identical!