I have a Williams brass Cab Forward on the bench, but the Mabuchi motor that is in it is a 3-pole Mabuchi RS-550, those don't handle back-EMF cruise at all well, AAMOF, not at all! That being the case, I decided to stick a Pittman into it.
I modified the Mabuchi motor mount for the Pittman mounting holes and got it mounted.
First problem was the only flywheel that I had with the correct 4mm extension nub was too large and was rubbing on the spring adjustment screw for the gearbox. Not having all the neat tools that Pat has for doing jobs like this, I just mounted the flywheel on the Pittman, clamped it in a vise, and used a file as it rotated to cut the flywheel down about 3mm in diameter, everything fits now.
Next, the nice SS driveshaft that I had sourced, courtesy of @harmonyards recommendation, was too long, and it was the shortest one they offered. I thought about cutting the shaft of the gearbox, but I decided that cutting a $2 driveshaft was smarter than an unobtainium gearbox that I could screw up! So, I hacked about 1/4" off the driveshaft and everything fits now.
Here's the rear power truck for the Cab Forward.
The difference in the Mabuchi RS-550 and the Pittman motor have been well discussed here, however I was amazed when I did some bench testing. The Mabuchi motor that I took out is on the left, and the Pittman is, of course, on the right.
Measuring the current draw at 5VDC was very illuminating! The Mabuchi draws SIX TIMES the current, and of course power, that the Pittman does at the same 5V input power! I even turned the Pittman down to 1 volt input, and it was still difficult to stall the motor grabbing the shaft. Of course, the Pittman runs dead quiet on 5V, the Mabuchi made all sorts of buzzing noise at 5V. I have high hopes that this will run a bunch smoother with the new motor.
This should convince any remaining skeptics of why we sing the praises of the Pittman motors! 
