Customizable 4-Stroke Inline-Four Engine Model - 0/180/180/0 Crank Phasing (Educational)

This is a CUSTOMIZABLE 4-stroke inline-four engine teaching model. It is built from a short TrueGeometry Playground recipe, so you can change the scale (S) and the radial clearance (CLEAR, in mm) and rebuild it in a minute. Each unit also has a pose block (a pose table is in the recipe comments) so you can freeze the crank at 0, 90, 180 or 270 degrees.

UNTESTED, UNPRINTED design study. Not an engine part. For display and classroom use only.

What it is: an open-frame crank-slider demonstrator, four units on one crankshaft, phased 0/180/180/0 like a real inline-four (each unit has a valve train display: two valves on the head, shown closed). Bore 30 mm, stroke 20 mm, rod 42 mm, with a hand wheel and knob.

How a 4-stroke works: intake, compression, power, exhaust over two revolutions. The valves are displayed closed (no cam drive), so this shows the layout, not the valve timing.

Files: the full assembly STL first (upright, main file), then component STLs: pistons, rod bodies, crank pins, wheel, knob, crankshaft, plus the Playground recipe (.txt) and a how-to-run guide (PDF).

Clearance check: a numeric capsule sweep over the full cycle (1 degree steps) found no moving pair closer than 0.3 mm, piston top to head at least 2.0 mm, and rod lateral movement at most 10 mm in the 15 mm bore. This is a capsule model, not the native /intersect (unreliable on rotated duplicates). World bounding boxes were also checked at vertex level.

Known compromises: the solid piston captures the wrist pin and rod (split it before printing if you want it to move); the back-plate stand and tie parts are design additions; the valves are a static closed-pose display; guide-ring rear rims merge into the back plate (fixed parts, cast-style join).

Mesh: native TrueGeometry Playground STL export, triangulated, no boolean operations. Assembly: 19,228 triangles, 200 x 98 x 51 mm.

Customize: open Playground, paste the recipe into the Script panel, edit the /set_var lines, press Run. The guide PDF walks through it. Full commands are also in the blog article.

Blog article (full recipe and details): 

TrueGeometry Playground: https://www.truegeometry.com/CoreWebGLTG/KBEAgentHTTP.html

AI-assisted: designed by a person with an AI assistant in TrueGeometry Playground.

License: CC BY-NC 4.0, made for students and teachers to print and study. Commercial use requires TrueGeometry's consent.

If you test-print it, please post a Make and tell us what worked and what didn't. It helps the community.