I have been working on improving the operating qualities of a Mantua 0-6-0 loco by replacing the OEM drive mechanism with a NWSL idler gearbox. Doing so requires a fair amount of milling of the frame, axle bearings, etc. to make room for the gearbox. I also had to add moldable lead in the loco shell and underneath the motor to replace the original motor angled frame and the metal I milled out of the frame.
I ran into problems with the NWSL gearbox: assembled according to directions, the motor overheated and the drive axle "loped," i.e, first fast then slow.
After a lot of disassembling/reassembling to cure the problem, I found I had to wrap the two worm gear shaft bearings with two layers of .002" Tamiya masking tape in order to get the gearbox halves to tightly grip the bearings without dragging on the worm. I also had to run one length of this masking tape down each side of the gearbox to space-out the gearbox halves to prevent the idler gear from dragging.
I haven't run this yet in the loco but with the motor/gearbox temporarily connected to an MRC transformer, the new gearbox has run much quieter and the motor much cooler than it has over the last several days of pretty much non-stop experimenting. This is meant to cast aspersions on NWSL products; they don't know why this was such a problem and they have been very supportive in offering solutions.