A slider crank turns circular input into guided straight motion. The guide must physically hold the slider, while a pivoting connecting rod accommodates the changing angle. In our reviewed Twin-Piston Engine the slider axis sits one module away from the crank axis, so ideal full travel is about 16.23 mm rather than the centred-slider shortcut of 16 mm.
Begin with a supported piston
Open the free Twin-Piston Engine instructions. This existing original level 3 model has an estimated 40-minute build time. Its motor drives an 8-tooth gear against a 40-tooth crank gear, and two crank pins move the pistons in opposing phases. We have reviewed the guide, retention and teaching instructions for this article; we are not announcing a newly invented catalogue model.
The credited mechi demonstration supplies one useful reference: a visible crank pin and connecting rod drive a linear slider. We inspected its opening mechanism footage and description, not a complete assembly tutorial. Our two-guide twin-piston arrangement is a different educational configuration. The creator's video is inspiration for the motion question, not evidence that our model has been physically tested.
A connecting rod alone cannot establish a straight path. Each piston block runs on two parallel guide axles. Their separation resists twisting about either guide; the guides also oppose sideways movement and tilt. Top bushes stop the guides dropping through their fixed bridges, and bottom half bushes keep the pistons from leaving the guide ends. The crankshaft is supported by posts and retained along its axle line. Read the axles explanation before mistaking a cross-hole connection for an axial stop.
Build and inspect in the right order
Follow the animated steps and free booklet in order. First make the frame and posts, then assemble the paired crank discs, pins and rods. The piston blocks temporarily attached to the rods are not ready to operate. Keep the crank still until the braced cylinder heads, both guide axles for each piston and all guide stops are fitted. The bracing idea explains why the head beams need triangles rather than a single pivoting attachment.
Leave power disconnected for the first complete turn. Hold the stationary frame, not a piston, and rotate the crank gently. Both guide axles must remain threaded through each piston at every position. Look for a rod catching a nearby beam, a guide creeping downward, a piston twisting or a crank axle sliding out. A clean screen animation does not excuse a missing bush in the assembled model. Stop and correct the connection before motor operation.
Predict the offset before checking
Measure centre to centre. One module is 8 mm. Crank radius is one module, connecting-rod length is six modules, and slider-axis offset is one module. A centred slider would travel twice the radius: two modules, or 16 mm. Here the offset changes where the highest and lowest positions occur. The ideal travel is √48 minus √24 modules, approximately 2.029 modules or 16.23 mm.
Try the lab with offset one. Predict travel, reveal the answer, then scrub a complete crank turn. Change only the virtual offset to zero and compare. Other offset settings are mathematical comparisons; they are not validated changes to the real engine's head or guide locations.
Interactive: Offset slider prediction. Predict offset slider-crank travel, scrub crank angle and compare centred geometry. Ideal rigid linkage, not physical measurements.
The lab solves the positive assembly branch of an ideal rigid linkage. It does not include hole clearance, elastic bending, friction or motor speed. Its position display is the piston pin height above the crank axis, not a measurement of the plastic block's top surface. Equal angle changes do not necessarily produce equal piston travel; the rod angle contributes to the result alongside the crank's vertical coordinate.
Make an honest observation sheet
Place a fixed ruler beside a guide without touching the moving pieces. Turn the crank slowly and find the two extreme piston-pin positions. A 0.23 mm difference between the centred shortcut and this prediction is small; a hand-held ruler may not resolve it. Record measurement uncertainty instead of claiming the model proves a tiny difference you cannot see.
| Trial | Predicted travel | Observed travel | Reference and uncertainty |
|---|---|---|---|
| 1 | 16.23 mm ideal | Record | Record |
| 2 | 16.23 mm ideal | Record | Record |
| 3 | 16.23 mm ideal | Record | Record |
Keep the ruler, viewing direction and pin reference the same for every trial. We have not supplied physical measurements. If results differ, check whether you measured the same pin centre, whether the guide supports moved, and whether the slider catches near an extreme. Increasing motor speed will not correct a geometric or support error.
Compare a circle with a real slider
The Crank Orbit Bench provides a useful comparison: its outer pin follows a circle and its straight-axis coordinate is only a projection. It has no physical sliding output. Read why crank projection moves unevenly to separate those ideas. Then return to the engine and explain what the connecting rod does and what the guides do. Those are different jobs, and both are necessary.
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- What holds each piston on a straight path?
- What is the crank radius in this model?
- Why is travel slightly greater than 16 mm?
- Does the ideal lab measure physical friction?
