Thomas the Tank Engine Monster Truck Outlaw from insultcomicdog on Vimeo.

Thomas the Tank Engine and the Grave Digger monster truck are popular in my house. We probably have 10 different Thomas trains of various sizes and vintages (wooden railway and other sets). This Christmas we gave our son a Hot Wheels Monster Jam Rev Tredz monster truck which turned out to be a surprise hit. Rev Tredz are 1:43 scale trucks with powerful friction motors that allow them to flip, climb and jump over pretty much anything.

What would happen if we combined two of Owen’s favourite things, Thomas AND monster trucks?

Rev Tredz are great toys to take apart because they are inexpensive and easy to find (2 for < $20 at Walmart). You could very easily take a Dremel and cut the truck body off and hot glue a Thomas shell on and call your project done. I wanted to try a more elegant solution, where I would design and 3D print an adapter that would sit between the monster truck base and the train shell. This adapter would allow the user to hot swap shells easily, allowing kids to convert their whole fleet of trains to monster trucks. The other goal of this project was for it to be non-destructive. Nothing should be altered permanently and all the toys should be capable of being reassembled back into their original states. Thomas Trackmaster sets are N scale (1:148 to 1:160) and have shells that are removable. You can avoid wasting the motorized bases, by buying spare shells online.

Motorized Thomas trains can be battery hogs. This conversion eliminates the need to constantly take your trains to the Steamworks to change batteries. The fly wheel friction motors of Rev Tredz are much more powerful than the stock train motors.

Build Log:

The Thomas shell I used is vintage and of unknown origin (ie I’m not sure it’s Trackmaster). It’s stamped Tomy Thailand, (Thomas) Limited 1992. It came with an Annie and Clarabel carriage and was part of a large set with dark blue track and suspension bridges. I believe it was a series that was produced between 1992-1998. I’m unsure of it’s scale, but it matches nicely with the Rev Tredz wheel base.

Designing and 3D Printing The Adaptor:

I modeled the adapter in SketchUp, and went through about 5 different iterations of the adapter before I finally arrived at a design that fit the Thomas shell well. I sacrificed ‘Crushstation’ Rev Tredz, because it’s red rims matched Thomas’ colour palette well.

Taking Apart the Rev Tredz:

The base is held together by 4 Philips screws. I removed two tires to have easier access to the bottom screws.

The truck body is connected to the base by two white plastic tabs that slide into channels in the base. The first time I removed the body I was able to put it back together without losing or damaging any parts. The second time I took it apart was a different story.

Two gears for the rear axle came loose inside the body, and I had to remove them to figure out their proper orientation. The rims for the truck are fixed, so you can’t physically separate the two halves. This makes the space to work in quite small and awkward. If your gears do shift around it’s not the end of the world. You don’t have to be a watch technician to fix your truck.

The gear layout is symmetrical. There are two gears for each wheel and a small gear and the large metal fly wheel in the middle. The black base has holes that anchor the gears.

Securing the Adapter:

I removed the white tabs from the original Rev Tredz body, and screwed them into my custom adapter using the stock screws that came with the truck. I made the mounting posts on my adapter as wide as possible with large bases, so they wouldn’t break under stress. FDM printing is great for prototyping but if even with a solid infill, narrow geometries can be weak. If this adapter design works out well, it could be molded and casted with a resin that would more resistant to breakage.

The adapter was then secured to the base, and tires were reinstalled.

The Thomas shell snaps on and off the adapter easily.

I’m currently testing with my son to see how durable this design is. These trucks take a beating and the simple friction fit locking mechanism might not be enough strong enough to keep the Thomas shell secured to the base.

Update: February 3, 2015

I picked up some spare TrackMaster Thomas shells on Ebay and decided to design and print a new adapter. While I like the scale of my existing Thomas Monster Truck, the shell itself is vintage and hard to find. TrackMaster Thomas shells are still produced today and are not as rare. Also it would appear, judging from the two different Thomas shells I got on Ebay, that the clip fasteners are universal allowing the shells to be interchangeable.

6 design iterations to get the right fit. The adaptors were printed in Orange ABS as a single part with supports.

The adapters got a coat of red paint, black paint for the bumpers and a coat of clear to protect the finish from chipping.

Three Monster Jam ‘Crushstations’ donated their wheel bases for this project. To quote Magnus Walker, “These are giving life to new cars. They are dying so someone else can live.” I need to figure out what to do with the extra lobster shells.

The packaging for Rev Tredz are mostly generic, with minimal badging for the individual car. The boxes are also very easy to open and repackage. With some minor customization to the box, this Thomas would look mint in the box! I think this would make and awesome custom gift for a toddler or any serious collector who’s into Thomas the Tank Engine and monster trucks.