🔥 Twin-Piston Engine: step-by-step build

See inside a car engine: two pistons race up and down in turns!

Inside a car engine, pistons shoot up and down hundreds of times a second, and a crankshaft turns all that up-and-down into round-and-round to drive the wheels. Your engine works the same way — only backwards: a motor turns the crankshaft and the pistons dance. Each piston hangs on a con-rod from a pin on a spinning gear. When one piston is going up, the other is coming down. A heavy wheel on the end, the flywheel, keeps everything turning smoothly.

• Level 3 of 5 (medium)
• About 40 minutes
• 22 steps, 71 parts
• Powered by: Motor
• Series: Mechanical Wonders

What you will learn

• Crank & Slider: A turning crank pushes a rod back and forth.
• Flywheel: A heavy spinning wheel stores energy.
• Electric Circuits: Electricity flows from the battery, through the motor, and back.
• Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
• Mounting a Motor: A motor must be pinned tight, or it spins itself instead of your wheels.
• Plugging In: Power Functions: Battery box → (switch or receiver) → motor. Plugs stack like bricks.
• Frames: A frame is a ready-made strong rectangle.
• Triangles Are Strong: A triangle cannot be squished — a square can.

Build these first

• 🔋 Motor Buggy: Your first car that drives by itself — with a forwards/backwards switch and headlights! (level 3 of 5, about 30 minutes, 20 steps)

Parts you need

• 1 × Open frame 5×7
• 8 × Blue long pin
• 4 × Split pin block
• 6 × Red axle-pin
• 2 × Beam 15
• 1 × Axle 6
• 7 × Bush
• 1 × Medium gear (24 teeth)
• 6 × Beam 7
• 1 × Beam 3
• 1 × Giant gear (40 teeth)
• 2 × Axle 4
• 4 × Long pin-axle block
• 10 × Black pin (grippy)
• 4 × Axle 9
• 2 × Double cross block
• 6 × Half bush
• 1 × Yellow-hub wheel
• 1 × Tiny gear (8 teeth)
• 1 × M motor
• 1 × AAA battery box
• 1 × Direction switch

Steps

1. Lay the 5×7 frame flat. Push blue long pins up into the 2nd and 4th holes of the back short end.
2. Press two split pin blocks onto the back pins, one on top of the other. Push a red axle pin into the middle of each block, pointing out the back.
3. Stand a 15-hole beam at the back and press its 2nd and 3rd holes onto the red pins. Push a 6-long axle through its 5th hole, then slide a bush and the 24-tooth crank gear onto the inside end.
4. Push a blue long crank pin into the gear one hole right of the middle, pointing to the front. Hang a 7-hole con-rod on it by its bottom hole, standing straight up, and press a 3-hole beam, the crank web, onto the pin tip so it points to the left.
5. Push a blue long crank pin into the web's left hole, pointing to the front, and hang the second con-rod on it. Slide the 40-tooth crank gear onto the pin tip by the hole left of its middle.
6. Push blue long pins up into the 2nd and 4th holes of the frame's front short end and press two split pin blocks onto them, one on top of the other.
7. Push red axle pins into the front blocks, pointing out to the front, and press the second 15-hole post onto them.
8. Hold a bush against the middle of the 40-tooth gear and push a 4-long axle through the front post's 5th hole, the bush and the gear.
9. Pistons: push a red axle pin into the top hole of each rod and press a yellow long pin-axle block on by its middle, end holes up.
10. Back cylinder head: pin a 7-hole beam by its middle hole to the inside of the back post's top hole. Pin a slanted 7-hole brace to the outside of the post's 11th hole.
11. Push a blue long pin through the brace's 6th hole into the end of the head beam: a triangle! Then pin the front head beam to the inside of the front post's top hole.
12. Front brace: pin a slanted 7-hole beam to the outside of the front post's 11th hole and push a blue long pin through its 6th hole into the head beam.
13. Push black pins into the 2nd and 4th holes of both top beams.
14. Press long pin-axle blocks onto the pairs. Push a 9-long guide axle down through each block's middle into its piston.
15. Slide a double cross block down each guide onto its block. Push a second guide through its other end into the piston.
16. Put a bush on top of every guide, so no guide can slide down.
17. Push a half bush onto the bottom tip of every guide, under the pistons.
18. Flywheel: put a half bush and the yellow wheel on the outside end of the back crank axle, and a half bush on the outside end of the front one.
19. Push a 4-long axle through the 8th hole of the front post from the inside. Slide a bush and then the tiny 8-tooth gear onto the inside end, meshing with the 40-tooth gear.
20. Push black pins into the 7th and 9th holes of the front post, pointing out. Press the M motor onto them: the pins go into the round holes above and below its orange output and the axle into the cross.
21. Power pack: put the AAA battery box on the table and click the direction switch onto the studs at one end, standing across the box.
22. Press the power pack up under the motor so the box's studs click into the motor's underside. Plug the switch cable onto the box, the motor plug onto the switch, and flip it!

Play time

Switch on and watch the pistons race. Flip the switch: does the engine run backwards? Take the flywheel off and turn the crank slowly by hand — can you feel the bumpy spots it smooths out?

Real-world inspiration

Slider-crank linkage — https://en.wikipedia.org/wiki/Slider-crank_linkage ; LEGO demos — https://doctorengine.info/post/lego-technic-mechanical-principles/