Lego Technic Steam Machine

A Lego Technic steam machine powered by two pneumatic cylinders engine, with valve gear mechanism, speed indicator, air control valve and building instructions.


Direct purchase of the instructions

Steam Machine (Pneumatic)
10,00  -Excl. Tax
A LEGO Technic steam machine powered by a 2-cylinder pneumatic engine, with valve gear mechanism, speed indicator and air control valve.

Preview of the instructions at the end of the article.


Since my last pneumatic mine, I wanted to build a Lego pneumatic engine with the new cylinders which are bigger. At the beginning, I wanted to build a Steam Traction Engine or a Locomotive to focus on the valve gear, but I wanted to use the new gear rack from the 42082 as the flywheel, nevertheless it was complicated to implement due to the scale. That is why I focused the work on the big steam machine like the mill steam machine which have a big flywheel at the center.



This machine is composed of a chassis, two pneumatic engines with valve gear, a flywheel which connects the both engines, a speed indicator display thanks to a governor and an air control valve. The valve gear is inspired by one on the locomotive but without the reverse mode. It enables to reduce the dead point on the switch and maximize the time on left or right position which increases the efficiency of the engine. It is composed of a main crank and connecting rod to the cylinder, plus a second crank which has a angular offset also connected to the piston rod with a linkage system which connects the both to the switch. This combination of the two cranks and linkage system enable to have a non-linear motion (you can see it on the video, the switch seems to jump from left to right and so forth).


The two engines are symmetrical, and are totally independent. The machine can work with only one cylinder or two, the flywheel connected the right engine to the left, using a gear multiplication of 5:1, it enables also to create a angular offset of about 90° between the two cylinders, which helps to make start the engine in any case (but create some vibration). Please note also that either the pneumatic cylinder, either the switches are not modified.


The speed indicator uses the same principle has on my old centrifugal tachometer. It is connected the the left engine, and the rotation speed raises the two arms which pull a lever which makes rotate the indicator. Of course this system do not regulate the air flow. Because in Lego, it doesn’t exist a proper valve, the only way it to pinch the hose which requires lot of force, force that can not be provided by small plastic arms which rotate. Maybe it is possible to make at a bigger scale and with a very soft hose.


The last equipment is the air control valve which is operated by the handle. It drives a worm screw on a 24t gear, with a small lever which pinches the hose. By modifying its section, the air flow is controlled, from totally closed to totally open to adjust the speed.


I wanted to make a working engine but also a nice detailed one. That is why I have chosen a color scheme for visual description, with yellow for pneumatic and moving parts, red for the engine chassis, and black for the rest. I have made also some details like the working area on a platform with the controls, and fences. At final, I find that the engine work smooth thanks to the big flywheel and the valve gears, and is very didactic and nice to see for people who like kinetic sculpture, that is why I have made a building instructions.


Pictures and Video

Purchasing the creations or the instructions

You can purchase either the whole creation on the website Buildamoc including the Lego genuine parts and building instructions in PDF, or just the building instructions on

buildamoclogoWant to buy this creation ? The steam machine have been indexed to Buildamoc and is for sale. The whole package included the Lego genuine parts, and the Pro rendering PDF building instructions.


Please note that the pro-building instructions is included in the whole package but you will need to have you own source or air (3 bar / 40psi). You can use :

  • 4x pump lego like this one
  • Use a lego compressor according to the specs
  • Use a electrical compressor like automotive compressor, industrial or electrical pump, you will need to put the air in the lego hose with . You can use a ball needle.
  • Use a hand pump with ball needle.

Please note that the color of the hose can vary, you only need to respect where each hose is connected and have to cut it if they are delivered in one piece. 

Building Instructions



There are not so many instructions for non-modified pneumatic engine, even less with the new bigger cylinders, that is why I wanted to create a high-quality building instructions for this machine, but also easy to set up. That is why the linkage mechanism is easy to build with no setup, each engine is independent (you can build only one if you want) and there is only one timing angle to respect on each engine (important angle are specified in the instructions). The building instructions are complete 204 step by step pages computer rendering building instructions at the price of 10€ (190 mo PDF). Here is the complete bill of material :

Want to know if you can build the Steam Machine ? The Steam Machine has been indexed to rebrickable, check it !

You can buy the instructions with the following button for 10€. You can also buy other building instructions and get an automated discount. You will get 15% off for 2 PDF, and 20% off for 3 PDF on the total), available for all the instructions for sale on

See it on the shop :

Steam Machine (Pneumatic)
10,00  -Excl. Tax
A LEGO Technic steam machine powered by a 2-cylinder pneumatic engine, with valve gear mechanism, speed indicator and air control valve.

After your payment, you will be redirected to a page for downloading your instructions, and will receive also the link to download by email. Please check the spam box and be patient to receive it. If you have trouble to download your copy of the PDF after payment or open the document, do not make a paypal dispute, please contact me using the contact form. The link to download the instructions expires within 24h. Contact me if you have lost your instructions. Note that you can print it, but not copy or distribute. Thank you for supporting my work 🙂

Here is some preview of the building instructions :


Some tips :
Please note that the instructions and the bill of material do not include the air source. You will need to use lego pump or a compressor as you want. I use this one in the video for instance. You can use different colors of hose, the colors shown in the instructions is only for viewing convenience, you can also use bigger length (or a bit smaller). If you have trouble to make the engine work, please check carefully the angular offset in the instructions on both engine and the correct building of the yellow linkage. If it still do not work, try to change the switch or the cylinder for a smoother one, or use silicon oil to help moving.

To know if you switch is smooth enough, you can build this building to test it (the wheel attached as to fall when you put the switch vertically). This is the configuration on the video, but it can also work with a less smooth switch (you have to help a little the wheel to drop), but no more stiff. If you have hard switch, considering modify them.