Any background information on how the gearbox originally broke?
my guess would be too much torque.
256-1 is a pretty hefty ratio and it would not be too dificult to get a lot of torque on that.
Do you have a picture of the application? Was there an additional ratio after this, to an arm (of what length and weight?)?
In this picture where you see the AM toughbox, there were (2) p60’s and (2) sprockets up top working together with a 3:1 sprocket reduction. That original setup was more sedate than what the AM toughbox with a 5:1 reduction is seeing. It’s much faster now and that’s a good thing.
What are you going to replace that CIM with?
We’re replacing 2 drive cims w/ banebot cimulators and bb775’s. Hopefully those will hold up for drive usage.
Are you using the FP? It would rock on a AM planetary on that toughbox.
I’m afraid of stalling/toasting FP’s. Last year our AM Toughbox kicker mercilessly stalled a CIM and beat the tar out of it but it never quit. So I’ll stick with Andy’s Toughbox and CIM combo. Andymark never let us down… good stuff.
i would not recommend using a FP in a situation like that, we did on our lift and toasted 4 before we got it marginally working, fisher prices really really don’t like being stalled.
We have an FP on our lift, but with neutral buoyancy and a screw drive, the motor is never under stall.
I’m more worried about the BB transmission than I am the motor under that setup.
It’s a legitimate concern, but seriously, with the kind of reduction you need for an arm, you won’t damage the FP at all.
The drivetrain is a much more stressful place than an arm for motors like the FP and RS-775. Seriously.
Well, as usual, categorical statements like that are not necessarily true.
It depends a lot on how things are designed, now, doesn’t it? After all, those FP motors are for Power Wheels… And what do they run on those Power Wheels, after all?
Plastic wheels that have no hope of stalling! 
With eight brothers and a propensity toward clown-car-ism, I can assure those wheels can stall!