James Watt's Steam Engine




Model by Mr. Kleinstein.
Small modifications and instructions by KaeroDot.

This is a working model of James Watt's steam engine. The model represents a beam engine, with an upright cylinder and a large flywheel. The to-and-fro motion of the piston inside the cylinder is taken up by the large beam, and then converted to a rotary motion by planetary motion gears. To make the piston rod on the end of the rotating beam travel in a completely straight path, a system of levers called Watt's parallelogram is employed. A sliding valve driven from the main crankshaft steers the intake and outlet of steam. The machine also includes a speed governor, an air pump for the steam condenser, and a feed water pump.

The model actually runs on a gentle vacuum, as provided by carefully approaching the steam outlet with the hose of a vacuum cleaner. There are 1152 bricks in this model. The build is quite challenging; the hardest part is adjusting all the parts to move really freely, and then centering the motion of the slide valve. When finished, the rigid frame stabilizes the engine so it can easily be picked up and handled without coming out of adjustment. See video on flickr:

The model contains also a fireplace, boiler and a heap of coal. The modeled steam engine would typically use a Cornwall-type boiler, which would be even longer than the machine (at the same scale). The engine frame would be at floor level.

All model files (.io, .ldr, .odt, .pdf) can be downloaded here:

The back side is transparent to see movement of piston and sliding valve:

Some other pictures:

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Name James Watt's Steam Engine
Designer Kaero
Designed 2018
Inventory 1146 parts
Theme Technic


  • 7 months, 2 weeks ago Hal99 Level 11
    Outstanding model! I'd really like to build it. Do you think it's possible to operate the mechanism with the Nico71's  pneumatic pump shown here: https://rebrickable.com/mocs/MOC-26524/Nico71/simple-pneumatic-hand-pump/#comments ?
    Thanks a lot for sharing the instructions!
    • 7 months, 2 weeks ago Kaero Level 9 Designed this MOC
      I do not know. But I think, that this steam machine is made for high flow rate of air, such as from vacuum cleaner. The Nico's pump can make higher pressure, but the flow rate is small. The reason for my opinion is that cylinder in this machine is built from bricks and there is quite a leakage in between. And if you build joints loose (not tight), you need only very small power (i.e. small pressure) to keep it running.
      Maybe if you would use some glue to remove most of the leakage, than maybe even a small flow rate would be ok. I believe any simple water-soluble glue would be sufficient. Still, the most of the leakage would be around the piston. In lego pneumatic system, they use seals, however in this steam machine there is no seal.
      • 7 months, 2 weeks ago Hal99 Level 11
        Ok, I'll give it a try, then. Thanks for your answer.
  • 1 year, 1 month ago Bertil-Hgl Level 4
    Fantastic model. The instructions are very clear and it is a big fun to build the model. Just a minor remark: the parts required for constructing the balancing beam are not in the parts inventory but are only specified in the bill of materials which is part of the building instructions PDF.
    • 1 year ago Kaero Level 9 Designed this MOC
      Thanks a lot for spotting the error. Whole submodel was incorrectly imported from ldr file. Now I have fixed it and checked all should be ok.
  • 1 year, 1 month ago Nautilus Level 14
    Nice model. Thanks for sharing the instructions.
  • 1 year, 1 month ago avb Level 12
    What a great model. My grandson is just going to love this. Finally, I have another use of all the parts (24121) I have.
    Love it
  • 1 year, 1 month ago Bolbuyk Level 18 MOC Designer
    This model is fantastic, both functionally and esthetically. Building instructions are great too, you've put a lot of effort in this. I like the details like the grease bulbs for example.
    • 1 year, 1 month ago Pioneer4x4 Level 13 MOC Designer
      I agree, I was going to say the exact same things.  It would make a nice display.
      • 1 year ago Kaero Level 9 Designed this MOC
        It makes wonderfull display :) Kids are all the time pushing the arm to run the wheel.
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This MOC also has a 3D digital file available (eg LDD, MPD, etc): 3D Model