On the left is pictured the prototype of a gearbox modification with a final gear ratio of 11.8:1. The
modifications described in this tutorial use existing ZipZap gears to achieve gearboxes of several different gear
ratios which are calculated to be faster than the standard ZipZap orange gears.With a variety of gears a multitude of ratios are theoretically possible. However, not all of those theoretical possibilities can be practically implemented. Gear location will offer some difficulties, but poor gear mesh will play a greater part in negating theoretical possibilities.
I would recommend the gearbox modification be accomplished in several steps, beginning with the upgrading of the ZZ MT itself, then the "remanufacturing" of a complete set of gear possibilities, and finally, the assembly of your chosen gearbox.
To remove the side play in the rear axle assembly, manufacture a c-clip from a no. 4 flat washer. By trial
and error, carefully cut the material from one side of the washer with a Dremel tool cutoff disk. The
c-clip is inserted on the gear side of the chassis immediately next to the nylon axle housing.note: Radio Shack markets a bag of small flat washers of various sizes, including both no. 2 and no. 4 washers.
Because the motor pinion gears are of differing diameters, it is likely the motor might need to be raised upward in the motor well to achieve good gear mesh. I recommend that several thin spacers should be built to have on hand to make these adjustments.
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On the left are shown three possibilities for motor pinion gears. Two are stock ZZ pinion gears--the 6-tooth
gear from a ZZSE and the 7-tooth gear which is standard on a ZZMT. The third gear is an 8-tooth gear made
from a ZZMT transfer gear.The 6-tooth gear is an easily-modified pinion gear from a ZZSE motor. Enlarge the motor shaft hole to 0.038 in. with a no. 62 drill.
Add images here to show how a small gear is cut free.
The 8-tooth gear requires a bit more effort to achieve. With a ZZMT transfer gear attached--small-gear-up--to a Dremel tool mandrel, slowly and carefully cut with an Xacto knife the small gear free from the large gear as the gear spins at a moderate speed. Next, enlarge the motor shaft hole to receive a short piece of small-diameter electrical wire insulation. Trim the insulation even with the sides of the gear, then press-fit the gear onto the motor shaft. In this example, the green pinion gear is shown with the insulation as a small-diameter red insert.
Although I have yet to try it, a 9-tooth pinion gear could be made in the same way as the 8-tooth gear was accomplished. While a 10-tooth pinion gear is likewise possible, successful gear mesh with the ZZSE transfer gear might not be possible.
Don't throw away the remainder of the ZZMT transfer gear; you'll be able to use it as a part of the axle gear modification described below.
note: Radio Shack markets an excellent set of micro drills; the set includes a pin vise.
Pictured on the right are three ZZSE transfer gears. They have been "remanufactured" for this mod by enlarging the
gear shaft hole. Even though you might not use all of these gears for your mod, it would be to your
convenience to have the set in your parts box.When enlarging the shaft hole, it is important to drill perpendicular to the gear face so as to prevent wobble when placed on the chassis. I'd recommend starting with a small drill and progressing to successively larger ones. Begin with a no. 58 drill (0.042-in. dia.), then a no. 56, then a no. 54, and finally a no. 52 (0.064-in. dia.). This method helps keep the drill perpendicular and centered with respect to the gears. If you have a small drill press, I'd advise using that, and at a slow speed.
The idler gear accepts an 0-80 machine screw to fix it to the chassis. Locate, drill, and tap the mounting hole in the chassis after the transfer gear is in place.
To position the idler gear, two no. 2 flat washers are placed between the gear and the chassis. Note that only the 18-tooth part of the gear is used in this example. The small part is not use and is not removed from the gear so that the gear may also be used as a transfer gear in other mods.
The transfer gear is mounted in the same manner as the idler gear--with two no. 2 flat washers as spacers between the gear and the chassis.
There are two different types of axle gears. One is a stock ZZMT axle gear--I've used a green one for this
example. The other type is a remanufactured combination of the 32-tooth side of a ZZMT transfer gear with a
ZZMT axle gear housing.Add images here to show how the gear is added to the gear housing.
| pinion | idler | transfer | axle | ratio | comments |
| stock 7-tooth ZZMT pinion |
orange 32/11-teeth ZZMT transfer |
orange 39-tooth ZZMT axle |
16.2:1 | stock gears | |
| modified 8-tooth ZZMT transfer |
any 18-tooth ZZSE transfer |
red 18/8-teeth ZZSE transfer |
green 42-tooth ZZMT axle |
11.8:1 | great gear mesh |
| modified 9-tooth ZZMT transfer |
any 18-tooth ZZSE transfer |
red 18/8-teeth ZZSE transfer |
green 42-tooth ZZMT axle |
10.5:1 | |
| modified 8-tooth ZZMT transfer |
any 18-tooth ZZSE transfer |
red 18/8-teeth ZZSE transfer |
modified 32-tooth ZZMT transfer |
9.0:1 | |
| modified 9-tooth ZZMT transfer |
any 18-tooth ZZSE transfer |
red 18/8-teeth ZZSE transfer |
modified 32-tooth ZZMT transfer |
8.0:1 | |
A gear ratio calculator is available elsewhere on this website for your use.
Put the transfer gear on the stock pin. Put a no. 55 drill in a pin vise and slide the idler gear onto the drill. With the two gears--the pinion and transfer--in place, locate the idler gear between them with what you determine to be good gear mesh. While the idler gear remains on the drill, drill a hole in the chassis at that location. Finally, with an 0-80 tap, thread the hole.
Remove the transfer gear, place two no. 2 washers on the stock pin, and replace the transfer gear. Likewise, place two no. 2 washers over the tapped idler gear hole, add the idler gear, and fasten the three items to the chassis with an 0-80 machine screw. Add the axle gear and wheel. The results should look something like the images below, depending on which gear ratio you have chosen.
