This tool slides in the disk brake caliper and over the brake disk to align the caliper correctly. You can then tighten the screws, insert the pads and if the pistons move freely and evenly, everything should end up nicely aligned.

The quick-and-dirty alignment methods with the installed brake pads and a light surface behind works, but it doesn't really align the caliper. If a piston gets sticky, everything changes and things start getting scratchy.

This model fits Shimano and Trickstuff brakes with 2 mm disks. I also included the Fusion360 file with a parametric model so you can adjust the dimensions to your particular brake.

For a nicer made model go to:

[https://www.printables.com/model/721341-shimano-road-disc-caliper-alignment-tool](https://www.printables.com/model/721341-shimano-road-disc-caliper-alignment-tool)

This may well have been the original that I saw a picture of somewhere that inspired me.