Torn between the OKMO B03 and the B04? You're not alone. Since both engines went live, it's the question we hear most: "They look so alike. Which one should I get?"
We heard you, so we put this guide together. Below you'll find what the two engines share, where they really differ, and a simple way to decide which one belongs on your shelf.
Choose the B03 "Hired Man" if you love the classic water-hopper farm engine and want the most traditional engine-show look.
Choose the B04 "Chore Boy" if you'd like simpler setup with no water to manage, and enjoy watching a spinning brass fan at work.
Two Farm Hands from the 1910s
In the 1910s, the Associated Manufacturers Company of Waterloo, Iowa, better known as Amanco, made small engines that did the daily work on family farms. They pumped water, ground feed and ran whatever needed power. The company named them after the help they replaced: the Hired Man took on the heavy work, and the Chore Boy handled the odd jobs.
The OKMO B03 pays tribute to the Hired Man, and the B04 to the Chore Boy. Together they show the two ways early farm engines kept cool: with water, or with air.
What the B03 and B04 Have in Common
Under the skin, the two engines are close relatives. If you're worried that one is "better" than the other, you can relax:
- Same 1/4 scale and the same size: 22.5 × 11.5 × 13 cm
- Same 3.6 cc horizontal single-cylinder, four-stroke gasoline engine (16 × 19 mm bore and stroke)
- Same working centrifugal hit-and-miss governor, running from 500 to 3,000 rpm
- Same fuel: gasoline mixed with 2T/4T oil at 25:1 to 30:1
- Same starting: flick the flywheel by hand, or use a drill
- Both powered by one AA battery for ignition
- Both arrive fully assembled and ready to run on a wooden base
So the choice isn't about power or quality. It comes down to one thing: how each engine keeps cool. That single difference changes how it looks, how you prepare it, and what you see when it runs.
B03 vs B04: Side by Side
| B03 "Hired Man" | B04 "Chore Boy" | |
|---|---|---|
| Cooling | Water-cooled: open water hopper, cools by evaporation | Air-cooled: cylinder fins + brass belt-driven fan |
| Signature look | The tall water hopper over the cylinder | Finned cylinder and mushroom-top stack |
| What you see running | On long runs, the hopper water warms and steams | The brass fan spins, blowing air over the fins |
| Before each run | Fill the hopper; top it up as water evaporates | No water needed |
| Running tips | Keep an eye on the water level | Run in sessions and check cylinder temperature |
| Weight | 1650 g | 1700 g |
| Launch price |
$399.99 |
$419.99 |
Watch Them Run
The best way to choose is to hear them. Turn your sound up.
OKMO B03 "Hired Man"
The B03 with its open water hopper.
OKMO B04 "Chore Boy"
The B04 with its finned cylinder and brass fan.
Which One Is Right for You?
Choose the B03 "Hired Man" if…
- You picture a farm engine with a water hopper on top
- You want the most traditional, engine-show look
- You'd enjoy watching the hopper water warm and steam on a long run
- You don't mind adding water before each run
$399.99$499.99
Choose the B04 "Chore Boy" if…
- You'd rather not deal with water at all
- You like extra moving parts to watch, like the spinning brass fan
- You enjoy the finned, more mechanical look of air-cooled engines
- You plan to run it in shorter sessions on your desk or bench
$419.99$519.99
Still Can't Decide? Bring Home Both.
Many collectors do. Side by side, the B03 and B04 tell the story of how early farms kept their engines cool, with water and with air. With $100 off each engine during pre-order, you save $200 on the pair.
Shop the B03 Shop the B04Make It Show-Ready
Whichever you choose, the ST-02 Steerable Engine Cart fits both the B03 and B04. It bolts under the engine's base with no modification, adds metal wheels, working steering and a wooden pull tongue, and gives your engine the classic look collectors use at engine shows.
Frequently Asked Questions
Is the OKMO B03 or B04 more powerful?
Neither. The OKMO B03 and B04 share the same 3.6 cc engine, the same 500–3,000 rpm range and the same hit-and-miss governor. The difference is the cooling system, not the power.
Which is easier for a beginner, the B03 or the B04?
Both arrive fully assembled and ready to run, so either is a good first hit-and-miss engine. The OKMO B04 has one less step, since there's no hopper to fill. The B03 simply asks you to add water before running and keep an eye on the level.
Does the OKMO B04 overheat without water?
No. The B04 is designed to be air-cooled, with cooling fins and a brass belt-driven fan, just like the original Chore Boy. As with any air-cooled engine, we recommend running it in sessions and feeling the cylinder temperature during long runs.
Can I run the OKMO B03 indoors?
Both engines run on gasoline and produce exhaust, so please run them outdoors or in a well-ventilated space. As display pieces, both are perfectly at home indoors.
When do the B03 and B04 ship?
Pre-orders for the OKMO B03 and B04 run until December 1, 2026, and orders are expected to ship by December 10, 2026.
Does the ST-02 cart fit both engines?
Yes. The ST-02 Steerable Engine Cart fits the OKMO B02, B03 and B04.
Still have a question about the B03 or B04? Email us at service@stirlingkit.com and our team will help you choose.
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