Why does a Shimano centerlock rotor have free play?
The cause is a missing aluminum shim between the centerlock lockring and the
brake rotor.
I removed the brake rotor from the defective front wheel. Then I removed a
brake rotor from a known-good front wheel. When closely inspecting the
lockrings, I noticed something the known-good front wheel has that the
defective front wheel didn't: there was a very thin aluminum shim between the
centerlock lockring and the brake rotor.
When I built the front wheel, having had no prior experience with disc brakes,
so I thought the aluminum shim is something that should be removed and the
centerlock lockring should be tightened without it.
Now I have swapped the lockrings and the brake no longer has free play.
The missing shim probably has been thrown out along with garbage, so I have
to purchase a shim somewhere.