4-Stroke RC Airplane Engines: How They Work, Sizes and Care
Four-stroke RC airplane engines fire once every two crankshaft turns and use valves (opened by a camshaft and pushrods) to let fuel in and exhaust out, just like a car engine. Compared with two-strokes of the same displacement, they produce less peak power but more torque at lower rpm, swing bigger propellers, use less fuel and have a deep, realistic sound that scale modellers love. They come as glow engines (from small sport sizes) and gas engines for large models, including multi-cylinder radials.
By Kyle HilsonUpdated: October 20263 min read
Four-stroke vs two-stroke
| Four-stroke | Two-stroke | |
|---|---|---|
| Power stroke | Every second turn | Every turn |
| Sound | Deep, scale-like | High-pitched |
| Props | Larger, slower | Smaller, faster |
| Fuel use | Lower | Higher |
| Weight and cost | Higher | Lower |
| Maintenance | Valve clearance checks | Simpler |
Engine types in general are covered in model airplane engines.
Sizing
Because four-strokes make less power per cc, kits list larger four-stroke sizes than two-stroke sizes for the same plane, for example “.40–.46 two-stroke or .52–.70 four-stroke”. Stay within the kit’s range. Makers such as O.S. publish recommended props for each engine; see OS model airplane engines.
Setup and break-in
- Mount the engine firmly; four-strokes vibrate less but still need solid mounts.
- Use the fuel type and nitro/oil content the maker specifies (four-stroke glow fuel often has lower oil).
- Break in with rich runs as the manual says.
- Tune the high-speed and low-speed needles.
- Check valve clearance after break-in and periodically.
Care
- Use after-run oil; four-strokes can rust in bearings and cam parts.
- Check valve clearances with feeler gauges.
- Keep the breather tube clear.
- Replace glow plugs when starting becomes hard.
- Store dry and free of fuel.
Radial engines
Multi-cylinder radial four-strokes are the ultimate scale engines for warbirds and vintage planes, sounding remarkably like the real thing. They cost more and need careful tuning, but nothing else gives the same effect on a scale model.
Who should choose a four-stroke
Pilots who fly scale models, trainers or sport planes at moderate speeds and value sound and realism tend to love four-strokes. Pilots chasing maximum power for weight, such as in pylon racing, prefer two-strokes. Many clubs also appreciate four-strokes because they’re quieter.
Checking valve clearance
Valve clearance is the small gap between the rocker arm and valve stem when the valve is closed. As parts wear or bed in, the gap changes. Too tight, and valves may not close fully, causing poor running and power loss; too loose, and the engine runs noisily. Remove the rocker cover, turn the engine until the valve is fully closed on the compression stroke, measure with a feeler gauge, and adjust with the lock nut and screw as the manual describes. Check after break-in and periodically after that.
Propellers
Four-strokes swing bigger props at lower rpm, which suits scale planes and gives strong pull for take-offs. Use wooden or engine-rated props of the size the maker recommends, balance them carefully, and never use electric-only props.
Many pilots find four-strokes easier to idle reliably, which makes landings smoother.
Always start the engine with the plane held firmly or restrained, and stay out of the propeller arc.
FAQ
Are 4-stroke engines better?
For scale sound and fuel economy, yes; two-strokes give more power for size.
Why are four-strokes larger for the same plane?
They make less power per cc.
Do four-strokes need special fuel?
Usually glow fuel with suitable oil content; check the manual.
What maintenance do they need?
Valve clearance checks and after-run oil.
More on this topic: the complete RC Airplanes guide covers the basics behind this article.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
