Looking to pick up your very first hit and miss engine? With RETROL and OKMO offering two popular horizontal four-stroke models priced within $30 of each other, many hobbyists find themselves stuck on which one to choose. It really comes down to your hands-on preference: if you love the challenge of assembling a larger engine from scratch and the satisfaction of tuning it to perfection, go for the HM-01; if you would rather skip the assembly and watch a neat little finished model start chugging the moment your package arrives, grab the B02. Follow your itch to build, and you will not go wrong.
RETROL HM-01
The one with more engine in it $461.97Running setup; engine cart $133.99 extra7cc, heavy flywheels, real power at the pulley. Sold as the engine only, so you configure it yourself: kit or assembled, red or black, with the base, ignition and four-wheel engine cart chosen separately. Two running modes and independent low- and high-speed mixture control give it the higher ceiling of the two.
Shop the HM-01
OKMO B02
The one that runs on day one $429.99Complete set, nothing else to buy3.6cc, 1/4 scale of an American Olds engine. Engine and base sold together as one set, ready to run, with a long miss cycle straight out of the box. A desk-sized display engine rather than a working one.
Shop the B02Who makes these engines
These are two well-known miniature engine brands that occasionally collaborate to release models together. RETROL covers the broadest lineup — from hit and miss and steam engines to V-twins — while OKMO specializes in precision stationary engines like the B-series.
Despite wearing different brand names, they share the same DNA: vintage engine designs brought to life with modern CNC machining. Instead of simplified toys, you get authentic mechanics mounted on solid wooden bases, complete with real mechanical ignition and evaporative water cooling. That is why these machines feel like proper mechanical models the moment you run them.
What a hit and miss engine actually does
If you have not owned one before, the name describes how it controls its speed. A hit and miss engine has no throttle. Instead, a governor holds the exhaust valve open once the flywheels are turning fast enough, so the engine stops firing and coasts — the miss. As it slows, the governor releases, the valve closes, and it fires again — the hit. What you get is one loud beat followed by a long quiet spin, over and over, and on a well-set engine that pause can run for ten revolutions or more.
Real farm engines worked this way before rural electrification, driving pumps, saws, and washing machines. That is also why they are so mesmerizing: every moving part of the cycle is completely out in the open, where you can watch the mechanics do their work.
Specifications
| RETROL HM-01 | OKMO B02 | |
|---|---|---|
| Displacement | 7cc — bore 20mm, stroke 22mm | 3.6cc — bore 16mm, stroke 19mm |
| Running modes | Hit and miss, or continuous four-stroke | Hit and miss |
| Form | Kit or ready to run | Ready to run |
| Power take-off | Split wide-face pulley, spring-loaded ball flywheel | Pulley for light belt work |
| Ignition | CDI, 6–7V pack (7.4V maximum), batteries not included | Mechanical contact, 1 × AA in the supplied battery box, battery not included |
| Mixture control | Fuel needle plus separate air bleed screw | Single needle valve |
| Lubrication | Total-loss drip system — piston reservoir plus two main shaft feeds | Three adjustable oilers — one piston, two main bearings |
| Piston rings | Silicone type, spare set supplied | Nylon, spare set supplied |
| Spark plug | 1/4-32 ME8, recessed in the head | 3/16-40, external |
| How it is sold | Engine only; CDI ignition, base and fuel tank, and engine cart bought separately | Engine and solid wood base together, one complete set |
| Finish | Red or black | Vintage red |
| Size / weight | 17 × 13 × 13cm, 1.8kg (engine alone) | 25.5 × 11 × 13cm, 1.8kg |
| Recommended oil | 4-stroke engine oil, mixed 25:1 | 2T/4T 10W50, mixed 25:1 to 30:1 |
| Price | $389.99 engine alone; $461.97 as a running setup; engine cart $133.99 extra | $429.99 complete |
HM-01 vs B02: what's the difference?
1.Size, weight and what it can drive
The HM-01 is nearly twice the displacement — 7cc against 3.6cc — with a bore and stroke of 20 × 22mm and two heavy flywheels. That mass is what gives it a slow, deliberate beat and enough torque at the pulley to actually turn something. One flywheel carries a spring-loaded ball, the other a split wide-face belt pulley, so belt work is designed in rather than tolerated. If you want to drive a line shaft, a generator or a workshop display, this is the engine that will do it.
The B02 has a take-off pulley and enough power for a light belt, but at 3.6cc it is happiest as a demonstration engine. It is also the physically longer of the two on the desk, at 25.5cm, because it models a full-length Olds engine at 1/4 scale.
2.Running modes
The HM-01 has a detent lever that holds the governor out of action. Released, it hits and misses; engaged, it fires on every cycle and runs as a conventional four-stroke. Under load through the pulley it fires more often, and you can watch the brass flyballs open and close as it changes. Two machines in one, and it demonstrates beautifully to anyone who has never seen how governing worked before throttles.
The B02 does one thing. What it does well is miss properly — roughly once in every ten revolutions, with a long quiet coast between hits. Plenty of models sold as hit and miss simply run continuously and never show the cycle at all, so this is worth something, but it is one mode rather than two.
3.Build it, or just run it
The HM-01 is the only one of the two you can buy as a kit, and the kit and assembled versions are close in price. The manual runs to around sixty pages of colour step-by-step illustrations with an exploded parts diagram and after-sales terms — closer to an engineering document than a leaflet. Screwdrivers, spanners, pliers and a small file are all you need, and you can expect to finish it in a single relaxed evening. You end up understanding your engine, because you are the one who set the timing and the fits.
The B02 ships assembled and tested, with no build to do and no build to enjoy.
4.First start-up
Both engines need the same four things before they will run: a battery, fuel, oil in the cups and water in the hopper. The B02 takes a single AA cell in its fitted battery box; the HM-01 takes a 6–7V pack in its ignition box. Neither engine ships with batteries, so pick those up at the same time as your fuel and oil.
The difference is the ignition. Because the HM-01 is offered as a kit as well as assembled, its CDI ignition is supplied as a separate set and is not wired in at the factory — even on the assembled version, that is simply how the production line is arranged. So your first job with an HM-01 is connecting the ignition. It is a handful of connections, and once they are made correctly the engine starts readily. Current kits ship with the upgraded CDI unit.
A quick note on fuel lines: an air lock is normal on any miniature internal combustion engine, not something specific to this one. Pull the line, refill it, and you are running again in a couple of minutes.
The HM-01 also gives you two mixture controls rather than one, so setting it takes a little longer the first time — which is the point of having them, as the next section explains.
The B02 asks none of this. It arrives as one set with the ignition already in place, tested before it ships, and there is nothing to set on the mixture, the governor or the timing before your first run.
5.Ignition and timing
The HM-01 uses a CDI unit with three timing wires, so timing is set electrically. The ignition has been revised since launch, and the kits shipping now carry the newer unit. It arrives set around 10–15° before top dead centre, which favours power at speed; move it closer to top dead centre if you want a slow demonstration engine. Spark is strong and stays consistent as the engine heats, and there are no contact faces to keep clean.
It also gives you two mixture controls instead of one — a fuel needle and a separate air bleed screw. That means you can set the low-speed and high-speed mixture independently rather than compromising between them, and tune the idle and the power delivery to the character you want. It is the single biggest reason the HM-01 rewards a tinkerer: the adjustment range is genuinely wider, not just fiddlier.
The B02 fires through a mechanical contact closed by a cam, off a single AA cell. It is what the originals used and it is adjustable by hand, but oil creeps onto the contact faces and they need wiping before each run.
6.Lubrication
Both use working drip oilers rather than decorative ones, which is not universal at this price. On many models the piston skirt never travels far enough to reach the oil hole and the oil simply lands on the conrod. On the HM-01 the piston genuinely comes out to meet it, and the transparent adjustable main oil cup lets you see and set the feed rate. The B02 answers with three adjustable oilers: one over the cylinder, two brass cups for the main bearings.
7.Fit and finish
- HM-01: several things that had not appeared on a Chinese model engine before — a counterweight cast into the wheel and aligned on both sides, a ball bearing on the cam follower, a steel connecting rod rather than aluminium or brass, steel pins and slots in the governor, and a discreet self-centring flywheel mount instead of a grub screw on the side. Machined faces show no lathe marks. Brass gearing throughout, a resonating exhaust, and no plastic anywhere except the ignition connector. Pick it up and it feels like a piece of engineering rather than a souvenir.
- B02: a Lunkenheimer-style mixer of the type fitted to real engines of the period, a crankcase oil splash guard, a brass fuel tank that screws in with a sight glass for the level, machined flywheels with an engraved starting-direction arrow, and turned brass caps on the oilers. It also sits still on the bench while it runs, which small hit and miss models often do not.
8.What you actually get for the money
The B02 is sold as one complete set: engine and base together, ready to run. The HM-01 is sold as the engine only, with the CDI ignition, the base and fuel tank, and the engine cart all bought separately. That is more decisions to make, but it also means you choose how far to take it — and it is the only one of the two offered in a second colour.
RETROL HM-01
Engine only, plus what you choose- HM-01 engine only, kit or assembled, red or black $389.99
- Solid wood base, brass fuel tank with sight glass, oil pipe $119.99
- CDI ignition, equipment box and spark plug $39.99
Running setup: $461.97 $549.97 if bought separately — the three are discounted together
Optional: the four-wheel engine cart, with brass-riveted spokes and stainless rims, mounts the whole engine and base on a wagon for display, $133.99. A 12V generator and a torque screwdriver set can be added on the same page.
OKMO B02
Engine and base, one set- Engine on solid wood base, ready to run, vintage red
- Ignition box, fuel tank, oil pipe
- Drill adaptor and pull cord
- Spare contact, O-rings, fuel hose, nylon piston rings
Complete: $429.99 Nothing else to buy
You supply a single AA cell, fuel, oil and water for the hopper.
Both engines run in Stirlingkit's seasonal promotions, and the discounts change through the year — sitewide codes, automatic reductions on individual models, and bundle savings that only apply when the parts are bought together. The figures above are the standard prices, so check the product pages for what is running at the time you buy.
Good to know first
On the HM-01, the spark plug is recessed in the head. A normal box spanner will not fit, and tightening it the first time is awkward. One small screw near the ignition mount is also difficult to reach with an Allen key. Neither affects running, but both are worth knowing before you are halfway through a build.
On the B02, the oiler needles above, and a battery box that sits slightly low so the lid can feel loose — a scrap of foam or double-sided tape inside solves it in a minute.
Questions before you buy
Which is the best hit and miss engine model for a beginner?
Either one works. Ask yourself whether you want to build and tune an engine or simply watch one run. The HM-01 gives you more engine for the money and teaches you the most: you wire the ignition, set the timing and tune the mixture yourself, and you finish knowing how your own engine works. If you would rather have it running on the day the box arrives with nothing to assemble and nothing to adjust, take the B02.
Hit and miss engine kit or RTR: which should I buy?
Only the HM-01 offers the choice, and the two are close in price. Take the kit if you want to build it — the illustrated manual walks you through it, ordinary hand tools are enough, and you will understand your own engine far better afterwards. Take the assembled version if you want the engine ready and are happy to spend your time tuning rather than assembling. Either way you still wire the ignition yourself, so the assembled version is not a no-setup option. If that is what you are after, the B02 is.
Are these hard to run if I have never owned a model engine?
No. There are four things to get right — a charged battery, clean fuel, oil in the cups and water in the hopper — and after that it is a matter of setting the mixture, which is one screw on the B02 and two on the HM-01. Nothing about either engine needs machining skills or special tools. The learning curve is in tuning for a slow, even beat, and that is the enjoyable part rather than the obstacle.
What fuel do you use in a model hit and miss engine?
Both run on unleaded pump petrol, 92 octane minimum, 95 where available, which gives the strongest firing stroke and the longest miss. White gas — Coleman, Shellite, naphtha — burns cleaner, smells far less indoors and starts more easily; the trade-off is a slightly shorter miss cycle. Add a small amount of two-stroke oil either way, enough to see a faint blue tint.
One warning if you pre-mix: white gas evaporates from the tank overnight and leaves the oil behind. Drain the tank after a session rather than starting the next day on what is left.
How slowly can these engines be made to run?
Slower than most people expect. Set up carefully, either engine will idle down to a handful of firing strokes a minute, with long coasting pauses between them — that slow, lazy beat is what most collectors are chasing. The governor sets the floor, and heavier flywheels help, which is one reason the 7cc HM-01 holds a low speed so steadily. Counter-intuitively, running slower usually means opening the mixture slightly richer rather than leaning it off.
Can a model hit and miss engine actually power anything?
The HM-01, comfortably. Larger displacement, heavier flywheels, a split wide-face pulley and a four-stroke mode for continuous output make it genuinely practical for belt work, and a 12V generator is offered alongside it. The B02 will turn a light belt but is built to be watched rather than worked.
Can you run a model gas engine indoors?
Less than you would think from the look of them. Run slowly on white gas, these are quiet enough to talk over and mild enough to run on a workbench. Petrol is noticeably stronger smelling. Neither gets hot at low speed. Run one slowly for an hour and the water in the hopper will still be close to room temperature.
Which to buy
Take the HM-01 if you want to get your hands into it — to assemble it, understand how it works, set the timing where you want it, tune the idle and the power separately, run it as a four-stroke, put it to belt work, and grow the display from base to cart. It is the larger and more capable of the two and it has by far the higher ceiling: there is always another adjustment to make. Do not let the word kit put you off, either. It is a fine engine to start on.
Take the B02 if what you want is simply to watch it run — a small, accurate, nicely finished Olds model that starts on the first turn and asks nothing of you.
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