Useful tool for centering disc brakes on rotors

This really could be gravel, road, CX or MTB but I’m posting it here.

With flat mount brakes and increasingly tight tolerances , getting rotors centred in calipers with no rub can be a pain. Normally it takes me hours and lots of rude words. Sometimes the classic hold the brake and tighten works but often not these days. I used to end up eyeballing it with a light or using the force until I get it just right.

Just got a new bike and had this issue and tried this cheap little tool that shims the rotor either side. Just jam it in, made sure the pads are covered evenly, hold the brake and tighten up. Seems to work first time if I’m careful and then no rub. Thought I would share since others might find it useful.


A tool like that works OK, but is nothing too special. Just a shim on either side between the pads and the rotor. Another option is just use two business cards, one on either side – does pretty much the same thing ( or any spacer the right size). The tool just makes it a bit easier.

Some brakes are better then others at pushing evenly and backing out correctly and don’t require any tool. One thing to watch for is that both pistons are moving equally when the leaver presses with no pads in ( and this can be more complex with 4 piston brakes!). If this is not the case you need to adjust and/or lubricate one piston. If brake is good quality and moving correctly then setup should be easy.

You are correct though and I have cursed and swore at brake adjusting many times. Be very careful if you are lubricating a sticky piston though – if you are not careful you can pop out the piston and that makes a big mess and can be a hellish time to get it back in!


P.S. Also check that your rotor is straight! A slight bend can make the job of eliminating brake rub really hard. I have a truing stand with a gauge you can use to check if the rotor is true – this can be useful. Bending bent rotors back into true is possible but can be difficult.

Overheat the rotors on a really long downhill and they will probably never be true again.

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Some good points there! The rotor point especially important. I have got brand new rotors in the past that were bent!

I’m not sure I have had a stuck piston with a new brake caliper but maybe I have been lucky?

I have not had much joy with business cards partly because they can be too thick and maybe they compress?!? Not sure. Since this is metal it is easy to jam in even if some force required ( which was needed with my new brakes which were incredibly tight) and thinner than a card would be. I’d agree any solid shim could be used but this is the right size and holds itself in with the cross piece. If it was $30 I would probably not bother ( and try and fudge something) but for $5 I think it has value.


Yes, that is what I’ve been doing for my disc brakes.


Stuck piston shouldn’t happen with new brakes.

It is more common with 4 piston brakes, and I have seen this with brand new 4 pistons. It is really common if you have been riding in Winter.

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