If I'm not mistaken the 27T pinion is marked because it's included in the kit. Brushed motors have a limited service life. I had suspected that my battery has failed me during a club race, but it was just a bad handout motor. It doesn't hurt to lube both motor bearings multiple times a day on these things.
You maybe right. In my experience, these mabuchi motors have failed me always in the same way.. the copper arm that holds the brush shears off and the brush is imbedded in the plastic of end bell or thrown out from the vent holes. This happened on two black can sport tune in my Sand Scorcher runner, so I stopped running that motor in it. With the torque tune 2 times as well, albeit the first time was when I had it geared (FDR of 3.7 or so) to have my son 7 at that time compete in our club class for VTA.. so it was geared very high and the motor died by days end.
I didn't expect it to happen in a TT cup car class using the Tamiya gearing (FDR 5.74). But to your point, I have ran this motor now for 5 race days for a total of 18 heats. Never cleaned it and never oiled the bearing. Something to consider for sure.
For next race I plan on trying 27t (6.16) to see if it makes a difference and see how long the motor lasts. Track is super tight so it should be ok.