Category Archives: Modifications to Sets

Simple Power Trailer Modification for LEGO Technic Tracked Crane 9391

lego-technic-tracked-crane-9391-built lego-technic-tracked-crane-9391-box

The Technic Tracked Crane (9391) is a great, inexpensive Technic set, but it doesn’t have any power. I wanted to do a simple mod to make the tracks spin.

Getting power to it is fairly easy if you’re willing to include the trailer. This method replaces the axle that rotates the upper portion with an axle that drives the tracks.

The first step is modifying the Tracked Crane so that the tracks can be driven and a trailer can attach to it.


First just replace the axle on the very bottom that runs parallel to the tracks with a longer one. You’ll probably also want to get rid of the black gear that spins the top portion of the crane (since this gear will constantly turn).

9391 Tracked Crane Trailer Modification - Bottom

Next attach a worm screw to the axle.


9391 Tracked Crane Trailer Modification - Worm Screw

Now take another axle and place it across the tracks to connect the back gear on each track to each other. On this axle, attach an 8 tooth gear which will be driven by the worm screw. You may need to firm up the axles with some other pieces.

9391 Tracked Crane Trailer Modification - Bottom

Finally, make a little trailer with a motor on it that can connect to the first axle. The only important thing here is that you make the connection the same height as the axle you’re connecting to on the crane. You may use a different motor than the one I used.

9391 Tracked Crane Trailer Modification - Trailer

Connect the trailer to the crane and you’re ready to roll (literally)!

9391 Tracked Crane Trailer Modification - Connecting 9391 Tracked Crane Trailer Modification - Connection