[Model Murdering]

That Riv looks sinister indeed, and I'm tickled to see them all shining  right side up like a car show! Thanx for indulging me. 

The Specialty chassis? Ya either love them, or ya hate them ... LOL.

Of all the things one can throw at them, dental hygiene is goes a long way to helping the motor lug all the extra monkey motion around. Everybody has some secret formula for lapping gear teeth, as well as elaborate methods to avoid getting "Anyonya", the gritty younger sister of the lapping process. There were once volumes dedicated to the topic in Cyberville.

For light duty (read especially plastic), I prefer to use a toothpaste base.

#1.  It's water soluble. No cost rinsing for the chassis, the bench top, or your clothes.
#2.  Your cars will smell minty fresh.

You'll need a dab of toothpaste in a plastic lid, and a swizzle stick. Add enough water drops from a common water bottle with the squeeze top, so as to create a light slurry. I use the stick to stir the mix, and daub it on.

DONT FORGET TO DO THE CROWN GEAR AND THE REAR AXLE JOURNALS, BILL!
As long as we're slopping around, I usually try to go for the value meal.

I use alligator jumpers from the bench test unit, so one can have have hands free; as well as cycle the power on an off to reload the cutting agent. Ya gotta stay right with it during the process. I keep some oil at the ready, as lubricity isnt always guaranteed with plastic gears and a cutting agent. It'll tell ya when it wants some!

What we're looking for, is listening. LOL! The auditory clue that you are done will clearly present itself when we hear the chassis pipe up clean and the RPMs increase beyond where the process was started. Often, WELL beyond; which is why we do it.

Simply put, the chassis will sing a better tune; so it's up to us to listen for it ... AND yer done! No point or benefit in going beyond the sweet spot.

For stubborn plastic gear sets, and most all metal sets, or any sets that have been scrambled up or randomized from their "ran in mating", I'll augment the slurry with some medium autobody rubbing compound. Best to keep it thinner and let it eat a while, rather than gobbing it; and heating up the wind. For the overly hard first gen Dash gear sets, some water soluble valve grinding compound was added to shut them up. True Grit. The exception, rather than the rule. No need for overkill unless warranted.

Rather than driving the gear train via some outside slave mechanism,  and then arbitrarily calling it good; simply because a certain amount of time has elapsed, has always mystified me since it was explained. Allowing the armature and gear train to tell us when it's optimized, was a tidbit an old fast guy imparted to me. So long as the resistance of the cutting agent is kept within the load capability of the motor, everything smooths out and we hit the sweet spot right on the button. Not because we think so, but because the armature sez so.

I have another simple alternative technique that works great for original T-jets, that I'll share here at some point.

Your enthusiasm inspires Ken. There will be cave time today!

Bill

PS: Here's an old trick that I usually have to kill people after I tell them. When one is brush tuning, running in a motor, or gear lapping; I use an old AM radio. Turn it on, dial it down towards the bottom of the scale and listen ... Low buck, low tech!