One thing that is important with the Slave motor on a Turbo Dyno is that it must be set to zero timing. Most 27T motors are fixed timing and since you are running the slave off the test motor it is running backwards which is not really good for it.

The best thing you can do is to try to find a large com Yokomo armature and use it in a can that has stand-up brushes. Set the motor to zero timing using a mod motor (adjustable timing) can.

If you can't find a mod Yokomo arm of the correct turn rating, you next best thing is to find one of those Reedy 19T quad mag motors. Like this: Reedy Pro 19T Quad Mag Brushed R/C Racing Motor #516 NIP NR BIN | #431622122

Then use that arm in a Trinity/EPIC, etc. mod motor can with stand-up brushes. Those reedy motors had the large comms. The benefit of this is that the slave motor lasts a lot longer before needing to be rebuilt because the large comm and narrow brushes eliminiates the brush overlap that causes arcing. You don't want to use that motor as-is because it has laydown brushes.

There are places that still do custom hand winding of brushed armatures (Holmes Hobbies comes to mind) but you really want a large comm blank if at all possible. And Zero timing.