A buddy and I were at the track today playing with the Revo and Jato and I blew my spare Revo tranny. This was the one I had not done the "pin mod" on, so I thought I'd snap a few pics as a sort-of "how to"...
The broken pin:
The Revo FOC tranny screw pin is a different color than the stock chrome-like tranny screw pin. I wonder if they are hardened and stronger? Well, I called Traxxas and they said "they might
be". Huh? Oh well, no real help there. They did say I should loosen the slipper to help control the torque, but then I'd lose much of the BL snap. Might as well drive a nitro!
Assortment of pins to choose from:
The Jato pin is too short to use anywhere in the tranny, but is included in the pic just for reference.
I noticed that whenever a pin broke, it was right where the threads meet the shaft, so that must mean that that spot is the most stressed. So, I modified two of the stock pins so they are about 7mm long each and then threaded both screws on each
side of the gear hubs so the shaft tips meet inside the shaft. Below is a pic of the modified screw pin:
When cutting the black FOC pins and the original chrome pins (same size, diff color), the black ones seemed to take longer to cut with the dremel, so I think they are harder. This method has an added benefit: Since both sides are now captured, if a pin does
break, the broken end won't fall out and mess up the other gears.
Picture showing the relative shaft diameters and gear hub thicknesses:
As you can see, each of the three shafts are different in diameter, so there is easy no "one fix for all" solution. The output shaft seems to be made of a softer metal so it is pretty easy to enlarge the hole. The input and idler shafts seem to be hardened so it is a PITA to drill. And the smaller shafts aren't big enough in diameter to hold up when drilled out.
Steps for each shaft:
Input (top) shaft:
Since 2 modified pins will be used: in the plastic gear hub, enlarge the hole on the other side of the gear hub (opposite the threaded side) using a 7/64" drill bit to allow for the threads of the second pin. Modify two of the stock chrome pins like in the picture above and screw them in. The chrome ones are OK to use here because the pins have less force being exerted on them due to the gearing.
Basically do the same thing as the previous step, but use two of the black
FOC pins instead. The pins here need the extra strength.
Since the output shaft is a softer metal, and has enough diameter, drill out a 2.5mm hole in the output shaft and the gear hub. There is no suitable "standard" size that properly equates to 2.5mm. Then drill a 1/8" (or 3mm) hole in one half
of the hub to allow for the larger pin threads.
Another alternative I haven't tried is to use either a cut allen wrench or drill bit shaft instead of the 2mm pins for the input or idler shafts. A 2mm wrench may be strong enough without needing to enlarge the shaft holes. You'd just need to file off the "points" of the allen wrench so it will fit. If done right, you could just use a bit of muscle to press-fit the pin in place. If it's too loose, you could epoxy or JBWeld the ends of the pin to keep them from falling out.
I torture tested this setup and it seems to be holding up very well.