Disk brake adaptor

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So I received my mag wheels and disk brake adaptor. I setup my forks to see how it all goes together and when the brakes are applied the adaptor moves forward about an inch before it catches.

In the pictures you can see there is a notch that goes into the fork dropout to prevent movement but it’s too small.

Any ideas on how to keep it from moving?

 

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I was thinking I could file the b**ch down and drill a hole -circled in picture- and add a clamp.