RETROL & OKMO N01: A 3.6cc Vertical Hit-and-Miss Engine

What It Is

The N01 is a working miniature of the vertical single-cylinder stationary engines that ran cement mixers and small farm machinery in 1920s America. It is a genuine internal combustion engine: it burns petrol, it has a spark plug, and it makes noise and heat.

At 1/5 scale and 3.6cc it sits at the larger end of the desktop hit-and-miss market. For comparison, the popular LETDIYMOC and M19 vertical engines are 1.6cc — the N01 has more than twice the displacement, with a 16 mm bore and 19 mm stroke. It weighs 1.5 kg and stands just under 20 cm tall on its base.

Being sold RTR matters more than it sounds. Most of the interesting hit-and-miss engines are kits, and a kit that will not run is a common and dispiriting outcome for a first-time builder. The N01 arrives assembled and, per RETROL, set up.

The Ignition Arrangement Is Worth Reading Twice

The specification lists both a "mechanical trigger-controlled igniter" and a 3/16-40 spark plug, which sounds contradictory until you realise what it describes.

A true period hit-and-miss engine used a make-and-break igniter — two electrodes inside the combustion chamber physically snapped apart to draw an arc, with no spark plug at all. The N01 instead uses a mechanical trip driven off the engine to fire a conventional spark plug from a 1.5 V cell. So the timing is mechanical and period-correct in feel, but the spark comes from a plug and coil.

That is the sensible engineering choice at this scale. A working make-and-break igniter in a 16 mm bore is a maintenance headache, and plug ignition starts far more reliably. Just do not buy it expecting a true igniter. If that distinction is new, our guide on how hit-and-miss engines work covers both systems.

Note also that the battery is not included and the recommended fuel is 92-octane petrol premixed with two-stroke oil at 25:1 to 30:1 — despite this being a four-stroke. The oil in the fuel is the lubrication system, supplemented by the drip oiler. Run it on neat petrol and you will wreck it.

Cooling and the Drip Oiler

The N01 uses an open evaporative hopper, which is the correct period arrangement and the more entertaining one. Water sits in a jacket around the cylinder, absorbs heat and boils off as visible steam; you top it up from a jug. There is no pump and no radiator.

Running it dry will damage the engine, so hopper watching is part of operating it. The trade-off is that the cylinder stays at a stable, self-limiting temperature no matter how hard the engine works, because the phase change soaks up heat.

The visible drip oiler on the crankcase is the other detail worth having. On the full-size engines this was how you could tell at a glance that lubrication was reaching the bearings, and it is one of the small mechanical rituals that make these engines satisfying rather than merely decorative.

The One Thing the Spec Sheet Does Not Tell You

Here is our reservation, and it is the same test we apply to every engine in this category.

The published specification never mentions a governor. There is no reference to a flyball governor, to a latch, to the exhaust valve being held open, or to a firing rate. For an engine sold as a hit-and-miss, that is a conspicuous silence — the governor latching the exhaust valve open is the entire mechanism that makes an engine hit-and-miss rather than just a small four-stroke. RETROL's copy describes a "characteristic intermittent-firing beat", which is a description of the result, not of the mechanism producing it.

This is very probably a documentation gap rather than a missing part. RETROL's earlier hit-and-miss engines govern properly, and OKMO's own video of the N01 is embedded above — watch and listen to it before you order. What you want to hear is one sharp bark followed by several quieter revolutions as the engine coasts, and what you want to see is the flyweights swinging out and something engaging the exhaust valve gear. If it fires evenly on every cycle, it is a nice vertical four-stroke, but it is not doing the thing the name promises.

We will update this page once we can confirm it either way.

How It Fits Against the Field

Against the engines in our best hit-and-miss engine models guide, the N01 occupies a specific gap.

The 1.6cc vertical engines — the M19 and the funnyndiy — are cheaper and do have visible flying-ball governors, but they are small and most are kits or part-kits. The Retrol OKMO 1/4-scale RTR engine is larger and also ready to run. The serious kits, LETDIYMOC and the Retro HM-01, reward a builder but punish a beginner.

The N01's argument is displacement plus RTR plus a period-correct hopper, in a vertical layout, from a manufacturer with a track record in this niche. If you want the largest ready-to-run vertical hit-and-miss engine currently available and you are not looking for a build project, it is the obvious candidate.

If you would rather build than unbox, spend the money on the Retro HM-01 instead.

Price and Availability

The N01 is $560 at EngineDIY in all three colours, and it is in stock rather than on pre-order.

A launch promotion ran at $460 with a $30 gift card, but that offer closed on 30 August 2026. If you received the affiliate or newsletter mailing advertising $460, that price has expired — check the current figure on the product page before ordering.

Three finishes are available at the same price: Engineering Red, Agricultural Green and Navy Blue. The box contains the engine, the drill-start adapter rod, the recoil rope starter and the manual. You will need to supply an AA battery, petrol, and two-stroke oil.