RETROL & OKMO 1115 Gas Engine: Classic Agricultural Power in Miniature

For decades, simple single-cylinder engines formed the mechanical backbone of small-scale agriculture across Asia. They powered walking tractors, irrigation pumps, generators, and other equipment where reliability, ease of maintenance, and straightforward mechanical construction mattered more than refinement.

RETROL & OKMO 1115 Gas Engine brings that mechanical tradition to the workbench in miniature form.

Rather than being a static replica, this approximately 1/7-scale engine is designed to run as a real four-stroke gasoline engine. Its exterior styling, horizontal single-cylinder architecture, lubrication system, cooling method, and mechanical controls all draw inspiration from the classic 1115 agricultural diesel engine.

The result is a compact working engine model that offers more than visual nostalgia. It provides a hands-on way to observe how traditional engine systems interact during real operation.

From Agricultural Workhorse to Working Scale Model

The original 1115-type engine belongs to a generation of straightforward agricultural power units designed around durability and serviceability.

Its horizontal single-cylinder layout made the engine mechanically accessible. Major components were easy to inspect, lubrication could be understood without complex electronics, and cooling depended on basic physical principles rather than sophisticated auxiliary systems.

The RETROL & OKMO 1115 preserves this mechanical character while adapting it into a miniature gasoline-powered engine.

Instead of diesel compression ignition, the model uses gasoline and spark ignition, making the miniature engine more practical at this scale. The fundamental experience, however, remains rooted in traditional agricultural-engine engineering: visible mechanisms, mechanical control, independent lubrication, and evaporative water cooling.

With a 16 mm bore, 19 mm stroke, and approximately 3.6 cc displacement, the model is small enough for desktop operation while retaining the architecture of a genuine internal-combustion engine.

A Real Four-Stroke Engine, Not a Display Replica

One of the most important distinctions is that the 1115 model is not simply built to resemble an engine.

It actually operates through a four-stroke combustion cycle.

During operation, the piston travels through intake, compression, power, and exhaust strokes while the crankshaft and flywheel convert reciprocating movement into rotary motion. This gives enthusiasts the opportunity to observe the behavior of a functioning single-cylinder engine rather than merely studying external components.

The large flywheel also plays an important mechanical role. A single-cylinder engine produces power intermittently, so the flywheel stores rotational energy between combustion events and helps maintain smoother crankshaft rotation.

This relationship between combustion pulses, inertia, and rotational stability is one of the fundamental principles behind traditional stationary and agricultural engines.

Mechanical Oil Pump Lubrication

Perhaps the most distinctive engineering feature of the RETROL & OKMO 1115 is its engine-driven piston oil pump.

Many miniature engines rely on simplified lubrication arrangements because of their small size. This model instead reproduces the logic of a larger agricultural engine by using a mechanically driven pump to deliver lubricant to important moving components.

There is no electric lubrication pump.

As the engine operates, the piston-type oil pump functions as part of the mechanical system itself. This approach serves two purposes.

First, it provides lubrication where friction and wear are most critical. Proper lubrication helps reduce metal-to-metal contact, control friction, and support more consistent engine operation.

Second, it makes the lubrication process part of the mechanical experience.

For enthusiasts, this is especially valuable because lubrication is often one of the least visible but most important systems inside an internal-combustion engine. Here, it becomes something that can be understood as part of the engine's overall operating logic rather than an abstract technical concept.

The use of metal piston rings further reinforces this mechanical approach. The rings help maintain compression between the piston and cylinder wall while also contributing to heat transfer and oil control.

Hopper-Type Evaporative Water Cooling

The cooling system follows another traditional agricultural-engine principle: simplicity.

Instead of using a radiator, circulating pump, electric fan, and closed coolant circuit, the 1115 model employs hopper-type evaporative water cooling.

Water is placed directly into the cooling hopper. As the engine produces heat, thermal energy is transferred from the cylinder area to the water. Part of that water gradually evaporates, carrying heat away from the engine.

It is a remarkably simple cooling method, but it demonstrates an important principle of thermal management.

Evaporation requires energy. When water changes from liquid to vapor, it absorbs heat, helping reduce engine temperature.

For full-size agricultural engines operating in environments where simple maintenance was essential, this type of cooling offered a practical advantage: fewer pumps, hoses, radiators, and moving components meant fewer potential failure points.

OKMO 1115 retains that same philosophy. A built-in drain tap also allows the cooling water to be released after operation without removing the tank or using additional tools.

Adjustable Mechanical Ignition

The engine uses a single-wire mechanical touch ignition system, replacing the more complex dual-wire arrangement found on some earlier configurations.

Beyond simplifying the wiring, the system also allows limited adjustment of ignition timing.

Ignition timing determines when the spark occurs relative to piston position near the end of the compression stroke.

If ignition occurs slightly earlier, combustion pressure begins building sooner. If it occurs later, peak pressure develops at a different point in the crankshaft rotation.

On full-size engines, ignition timing can influence starting behavior, throttle response, operating temperature, efficiency, and smoothness.

OKMO 1115 model allows users to make small timing adjustments manually, providing an opportunity to experience how ignition timing affects engine behavior.

This is one of the features that moves the model beyond simple operation and into mechanical experimentation.

Why the Horizontal Single-Cylinder Layout Matters

The horizontal single-cylinder layout is more than a visual reference to vintage agricultural machinery.

It also makes the mechanical architecture easier to understand.

In a compact multi-cylinder engine, many systems are hidden inside densely packaged castings. In a traditional horizontal single-cylinder engine, the relationship between the cylinder, crankshaft, flywheel, lubrication components, fuel system, ignition system, and cooling structure is much easier to follow.

That makes this type of engine particularly suitable for mechanical demonstrations.

For collectors, it provides visual character.

For hobbyists, it provides a working machine that can be started, adjusted, observed, and maintained.

For engineering education, it provides a compact example of several core engine principles operating together in one system.

Starting and Operating the Engine

RETROL & OKMO 1115 is supplied in RTR, meaning the main engine assembly is already completed.

Starting is performed with an electric drill rather than a hand crank. Once the engine reaches sufficient rotational speed and the ignition and fuel conditions are correct, combustion can sustain operation.

The ignition system requires 1.5 V power from one AA battery, which is not included.

The recommended fuel is gasoline of 92#, while the lubrication system can use suitable engine oil such as 5W-40 2T/4T oil, according to the supplied operating recommendations.

Because it is a genuine combustion engine, correct preparation remains important. Fuel, lubrication, cooling water, ignition condition, and mechanical settings should all be checked before operation.

Compact Size, Full Mechanical Character

Despite its relatively small dimensions of approximately 14 × 11 × 11.5 cm, the engine incorporates several systems normally associated with much larger internal-combustion engines.

Key specifications include:

What makes those numbers meaningful is how much mechanical functionality is concentrated into that footprint.

The engine combines combustion, ignition, lubrication, cooling, compression, timing, and flywheel energy storage in a form small enough to operate on a workbench.

A Model for Collectors, Builders, and Mechanical Enthusiasts

The appeal of RETROL & OKMO 1115 lies in the combination of historical styling and functional engineering.

For collectors, it represents the visual language of traditional agricultural machinery.

For engine hobbyists, it offers a real combustion engine with mechanical systems that can be observed and adjusted.

For mechanically minded users, the oil pump, evaporative cooling system, metal piston rings, and adjustable ignition provide more depth than a conventional display model.

And for educational demonstrations, the exposed structure helps connect individual components with the larger operating cycle of an internal-combustion engine.

More Than a Miniature Engine

RETROL & OKMO 1115 Gas Engine is ultimately a miniature interpretation of an engineering philosophy built around simplicity.

Mechanical lubrication instead of an electric oil pump.
Evaporative cooling instead of a complex circulating system.
A large flywheel supporting a single-cylinder power cycle.
Mechanical ignition timing that can still be adjusted by hand.

These features reflect the same priorities that made traditional agricultural engines so enduring: straightforward construction, visible mechanical logic, and systems that can be understood through direct interaction.

For anyone interested in vintage engines, agricultural machinery, mechanical engineering, or working engine models, the 1115 offers something increasingly rare—a machine small enough for the desktop, but complex enough to teach how a real engine works.