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I would like to change my brake disc. It is a Shimano SM-RT10-M that I will switch with this one: https://www.probikeshop.fr/disque-shimano-rt30-center-lock-argent/159249.html

I don't have the tool required and I am considering buying this one: https://www.probikeshop.fr/demonte-cassette-unior-shimano-aa67-5-4/125721.html

How can I be sure this tool is compatible with these brake discs? I read some stories that some tools could be too big.

EDIT: Here are some pictures of my front wheel. enter image description here enter image description here

Could you confirm I select the right disc?

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The Shimano cassette tool you linked is not suitable for the style of rotor you have now, or intend to purchase, though it is compatible with the alternative style of Shimano rotor lock ring you are not considering.

The tool yours requires is a Shimano Hollowtech II external bottom bracket tool which has 16 notches and 44 mm diameter. We see on the product page for TL FC-36 that it also suits disc brake rotor lock rings. Shimano make some similar looking, but smaller diameter, tools which would not fit, but they do not mention Hollowtech II in the product description, so that is your key phrase.

I see tools from other manufacturers state their Hollowtech II tools also fit 16 notch brake rotor lock rings (e.g. park BBT-9 states "Also fits the rotor disc lock rings using 16 external notches")

Using the tool is straightforward and intuitive, it fits on the 16 notches of the lock ring - the silver circle with Shimano stamped on it - and turns anti-clockwise to loosen, it should be tightened something like 40 Nm (refer to manufacturer's instructions).

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As you have found, Center Lock rotors are tightened and untightened by using a Shimano cassette tool. The tool you linked is an example of it.

You need to use the tool with a separate wrench. That could be an adjustable wrench or 1/2" socket wrench.

If you buy the correct tool, it won't be too big. There's only one size of Shimano cassette lock ring tool. The separate wrench can't be too "big" (long) either, but it can be too short in which case you don't have enough leverage to untighten the lock ring.

Edit: there are some lockrings that use a bottom bracket tool instead. You can distinguish them by them having teeth on the outside whereas normal lockrings have teeth on the inside. I presume these bottom bracket tool lockrings are sometimes used because some front hubs have a 15mm thru-axle which would be bit too large for a normal lockring. There's no reason to use bottom bracket tool lockrings on 12mm axles. Although rotors come with a lockring, it may be of the incorrect type (example: you bought a rotor with a normal lockring but have 15mm axle) so you may need to purchase a new lockring separately. Lockrings are sold separately, too.

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    The rotor in the picture is to be fitted with a Hollowtech II bottom bracket tool, not a cassette locking tool. Jun 11, 2022 at 16:47
  • There's no reason to use bottom bracket tool lockrings on 12mm axles Sure there is: to match the lockring you have in the on the front wheel with a 15 mm axle so that you don't have to travel with an extra tool.
    – Paul H
    Jun 14, 2022 at 23:28
  • Although not necessary in this case, yes, there's sometimes a reason to use this type of lockring with 12mm axles—one of them is for hubs such as Chris Kings where a bearing adjustment cone would make it impossible to use the narrower type.
    – karolus
    Oct 4, 2023 at 3:02

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