I stumbled across this thread after searching for a manual for the Dolphin. I don't particularly need one but it would be nice since I'm restoring the one I bought back when I was a kid. I'm actually restoring a Tamiya Blackfoot as well, and doing a "resto mod" to a Radioshack truck which is surprisingly good quality for not being hobby grade.
Now... the gearbox. My oh my this gearbox. I read through Lowgear's posts and I feel his pain! I had the exact same problem. I probably spent 15 hours troubleshooting, and took it apart at least 50 times! In the end I got it to mesh perfectly. Here's my story. Feel free to skip to the solution.
I ordered a bunch of new parts including all new gears off eBay and plunked them in the gearbox thinking it would just work. It did not. The two gears pictured bound terribly every half turn or so.
They would mesh fine when only one half of the gearbox cover was in place, but not both. I decided to do some measurements and calculate the pitches and determine the center distance between the gears - that is, the distance at which they should mesh perfectly. I came up with 21.51mm (0.847"). I carefeully measured the half of the gearbox that supports both gears and found Aristocraft spaced them at 21.47mm (0.845") which is perfectly fine. Within measurement error.
But the smaller of the two gears is supported by the aluminum plate which runs between each half of the gearbox. This plate has room to move around but I measured a center distance of 21.1mm (0.831") which is 16 thousands off! Now to see if this was actually the problem, I took a piece of scrap aluminum and drilled two holes at 21.5 and two holes at 21.1mm and put the gears on. Sure enough they meshed fine at 21.5 but at 21.1 they bound periodically just as they did in the gearbox.
This is where I made the mistake of thinking I could file the gears down to mesh. I spent probably 2 hours hand filing all the teeth on one gear only to end up with a marginally better fit. What a waste of time and effort.
The soution:
During assembly, I noticed that if you took the aluminum center piece and pushed it up towards the motor mount (see image), the teeth would mesh a lot better but upon tightening the screws it would get worse. So I took the plate out and redrilled all the screw holes to 9/64ths and filed out both of those hexagonal holes to allow more wiggle room. Once you've done that, upon assembly you can push the aluminum plate upwards and tighten the screws down. This spaces out the two problematic gears and allows for a perfect mesh.
And one more thing to note, the differentials tend to bind really badly as well. I found I had to file down the backside of each of these gears which made a huge difference in how freely they spin.
You'll also notice that mine are cracked to hell and back like Lowgear described. Interestingly enough the front was fine.
Here's a couple beauty shots of my completed gearbox. I've used ball bearings throughout and will continue to do so for the front diff assembly and wheels. I remember this thing being particularly drag prone as a kid, compared to the Blackfoot. It seemed like it was working harder to do less. Not surprised given the complete lack of ballbearings and gearbox issues.
Actually, one more thing. In my second picture you can see a hole in the gearbox which allows you to access the set screw to change the pinion gear. Does anyone know if there's supposed to be some kind of cover for it?
Looking for parts!
-Two springs for the shock absorbers
-The intermediate gear from the gearbox (The larger of the two which bind)