One thing that often seems to be a gotcha in building complex drive systems with Legos is that the motors lack power. (The new Power Function XL motor looks to be available only in the Bulldozer kit.) So i decided to see what I could build for cheap. I found a suitable motor on the Sparkfun sight for about 10 bucks and ordered it. The Tamiya kit features dual motors (that function independently) and seems to be about the right amount of power for my Lego needs. The only downside is that it isn't a perfect mate for the Lego system. The purpose of this Instructable is to show one way to adapt it for use in Legos construction. Caution: More powerful motors draw more current and may not work with Robotics controllers.

Step 1: What You'll Need

Here's the Lego parts we'll use. Shown are the 3749 Axel Pin and the 6536 Axel Joiner Perpendicular. Pick a suitable mounting plate as well. As you can see, the one I used is 6 x 8. One motor has already been modified; I'll go through the modification of the other motor in this instructable.

Parts list:
Tamiya Twin-Motor Gearbox kit, Item 70097. Includes the mounting bolts. Available from Sparkfun.com.
6x8 Lego Plate.
3749 Axel Pin
6536 Axel Joiner Perpendicular
Lego Electrical Connector Cable
Epoxy (I used JB Weld â€“ regular, not quick setting.)
Lacquer thinner to clean parts before gluing. (I usually do this to remove mold release, oils, etc. However: Caution: Do this outside! Lacquer thinner is *extremely* flammable.)
ABS, PVC, & CPVC pipe cement. (This was suggested on several web sites to glue the Legos. Caution: Do this outside! The pipe cement is *extremely* flammable!)

Step 2: Mount Motor to Plate

Build the Tamiya kit following the Standard Speed, B Type instructions. This leaves the drive axles in the best position for mating with Legos and still provides plenty of speed and torque.

Mount the motors to the mounting plate using the screws included with the Tamiya kit. Drill a hole from the back through the center of the round Lego feature. These seemed to line up perfectly with the holes in the motor. Grind or file the heads of the screws so they fit and are flush. The output shafts will stick out appreciably.

Step 3: Trim Motor Shaft

Next, mark and cut the Tamiya output shaft. Mark the shaft at the edge of the mounting plate. This will give you the length you need when the Lego parts are installed. Putting an ink line on the shaft with a Sharpie and using an Xacto knife to precisely mark it works well. The picture shows where to align the shaft with the edge of the mounting plate.

After you mark the shaft, slide it out of the motor assembly. You may need pliers to do this. If you do this carefully, the Tamiya gear will stay in position and you can use a small wire or toothpick to hold the gear that was on the shaft in place. Use a hack saw to cut the shaft and a grinder or file to dress the end.

If you try sliding the Axel Pin onto the Tamiya shaft by itself, it almost seems to fit. But it won't stay on and soon begins to slip. My first idea was to epoxy the Axel Pin by itself to the shaft, but the barrel of the Axel Pin wasn't strong enough and the epoxy inside it wasn't thick enough. The barrel flexed, the epoxy cracked and the pin came loose and fell off.

Step 4: Trim Joiner for Reinfocement

My solution was to use half of the Axel Joiner to strengthen the barrel of the Axel Pin and hold additional epoxy. The Axel Joiner is cut so that only the part that fits over the barrel of the Axel Pin is preserved. Use a file to clean up the remaining part of the Axel Joiner so it's nice and smooth. Any burrs will interfere with rotation of the finished shaft.

Step 5: Glue Axel to Shaft

Caution: Do this step outside! Lacquer thinner is *extremely* flammable. Clean the motor shaft, Axel Pin and the cut Axel Joiner pieces with lacquer thinner. Next, coat the inside of the Axel parts with epoxy. (I used JB Weld â€“ regular, not quick setting.) Be sure to work the epoxy down into the sleeve on the Axel Pin fully to get air bubbles out. Put the Axel Pin sleeve into the cut Axel Joiner and slide them both onto the motor shaft. Wipe away excess epoxy and clamp the assembly until it sets. Be sure the Axel Pin is straight on the motor shaft. Don't overdo the clamping force; you just want to keep the shaft from sliding out. After the epoxy has set for a few hours you can use the Xacto to scrape away any remaining adhesive that's not where you want it.

Step 6: Mount Power Connectors

While the epoxy is setting, you can prepare the power connectors and glue them to the motors. Use a connector cable from Lego. Cut the wire leads to length, strip them and tin them. Sand the bottom of the blocks until the center touches the sand paper as well. This is important since the center also contacts the motor frame and strengthens the glue joint. Use your Xacto knife to remove the lettering on one side of the motor frame â€“ the other side is OK. As suggested on several web sites, I used â€ABS, PVC, & CPVCâ€ pipe cement (local plumbing supply or hardware store) to glue the Lego connector blocks to the motor frame. Works great! Caution: Do this step outside! The pipe cement is *extremely* flammable! I let a drop of the cement fall on a piece of paper, then used a toothpick to apply it liberally to the motor frame. Set the connector block in place and hold it for 30 seconds or so. Do one side at a time and let the glue set about an hour before doing the other one. Solder the leads to the lugs on the motor when the glue is completely set.

Step 7: Power Your Lego Creations!

When the epoxy sets (24 hours or so), slide the motor shaft back into place through the gear and bushings. Repeat for the other motor. That's all there is to it. You've now got two powerful Lego motors for not many bucks. You may need to use the Lego battery box to provide enough power to your motors, since they draw significant current.