Polini Thor 250 Engine Review — The best ultralight engine we’ve seen

There’s a general lack of information on state of the art ultralight aircraft engines. And there’s a minefield of mis-information on what’s been happening with small two stroke engines in ultralight aircraft. Many believe that two-strokes always seize and many also believe that the only engine to use is a Rotax.

A lot of that is chewed up baloney.

I can’t clarify every issue that’s out there — and I want to establish some positive vibes for a few of the engine manufacturers that still stand strong today — and I want to precisely distill what I’ve found with the Polini Thor 250 into this small collection of words and photos.

First, the background vibes on the current crop of engines. They are good engines. Each has positives and negatives.

— Hirth is making an exceptional 50HP engine — the F23. It is what I refer to as a stump-puller. It has incredible low end torque and is ideal for ultralights with a lot of drag and a need for acceleration and power. It yanks our Belite Sealite floatplane out of the water, even in hot and high and calm conditions. It makes runway length an optional requirement. It is reliable; it is available with electric start, fuel and oil injection, and it is a great value for the money. We’ve sold a fair number of them; but it is too much engine for many of our customers. And it uses a lot of gas (think a mininum of 3.5 GPH; 4 might be more realistic.)

— 1/2VWs continue to be the ‘go-to’ engine for pilots who want four strokes. They don’t have a lot of bottom end torque, but they do have a great reputation for easy to service, reliable operation with a low fuel burn. They are heavy, and ultralight customers fight the tradeoff between weight, legal FAR part 103 operations, and the classic boxer look (which these engines share the Hirth F23). They are a great choice if you are comfortable with the weight and can stand a little longer takeoff roll.

— Compact Radial Engines (CRE) continues to provide the MZ-201 (baby brother to the MZ-202 helicopter workhorse engine.) I’ve always enjoyed the E-ticket thrill ride associated with an MZ-201 takeoff. (Any early Disney theme park fans in reader-land???) The MZ-201 is economical; relatively smooth running; and a jolt in the butt when you hit the throttle.

But for the average Joe, these engines are just a little marginal. Consider:

— The F23 from Hirth is too much power and too high fuel consumption.
— The 1/2 VW is very heavy, and not a great performer in climb.
— The CRE MZ-201 is a little harder to cool, fairly smooth, but not very up to speed on current engine technology.
— And if you’ve noticed that I haven’t mentioned Rotax, it’s because they haven’t pursued ultralight engines in years; they’ve discontinued engines; and they have found a gold mine in ‘912’ and ‘914’ class engines. That’s where their heart is. Leave them alone. Let them be.

If I was to dream about a very small aircraft engine, I’d make up a wish list with improvements on all of the above engines. The wish list would be this:

— The engine must produce an honest 35+ HP.
— It must be very lightweight.
— It must be from a major engine manufacturer. No startups, sorry.
— It must be smoooooooooooth. Like a 4 cylinder GA engine.
— Dual ignition. It’s got to have it.
— Reliability. I’ve got to be comfortable flying cross country in it.
— Fuel economy. I want to be able to loiter at 2 GPH, so that my 5 gallon fuel tank will give me 2 1/2 hours of fun. (Or 2 hours with a 30 minute reserve, to be a little more conservative.)
— It needs to look good.

I found that engine. It was from a major engine company called Polini, made in Italy to Italian standards (think recent vintage Ferrari), and it met every requirement I’d listed.

I was at the Aero-Expo in Friedrichshafen Germany, wandering around the exhibits of the various companies. I walked by a booth and was stopped by a row of engines which looked like they were the right size to be ultralight aircraft engines. One in particular caught my eye: it had dual ignition, a water jacket over the cylinder (water cooled!), gear reduction, a very small size, and the information showed that it developed 36.5 HP (27300 Watts). The indicated weight was less than 50 pounds! And it had an internally counterbalance shaft, for smooth running!

After a few months of reflection, I ordered one of the engines as a big test. Would the weight indications be accurate? Would the horsepower be real?

The engine arrived; we pulled it out of the box; and it looked like this:


It had been packed into a shipping box, whilst mounted on a steel frame. Sweet. I’d never seen anything like that before.

It had the following goodies in the box:

— a starting battery
— machined aluminum engine mounts, with rubber bushings
— a pre-wired electrical panel, with an engine start switch and dual mag controls (more on that later)
— a radiator (more on that as well)
— all the hoses, bolts, washers & nuts
— propeller hub plate


Very impressive. Very, very impressive.

The engine installation was straight forward; a few weeks later, we had it installed on our ProCub Lite demonstrator airplane. We chose to machine up a couple of extruded rectangular cross bars for the engine mount; if we weren’t into making so many lightening holes in our engine mount it would have gone even quicker. The photo belowIMAG0279-2-1 shows the engine mount, along with the impressive Polini fit & finish, all on our ProCub airplane.

One day while I was hanging around at the shop, I decided to test the strength of our Polini motor mount by resting nearly 250 pounds of engine and human blubber on the mount. I formally documented the results in the following picture:


And I should note a few things about the process of getting the engine mounted on our airplane. Machined aluminum standoffs were included, along with rubber bushings. We just bolted those to our engine mount. You can see them in one of the photos. (Albeit with a caution: ALWAYS install a heavy safety wire or strap in case a bushing breaks. And change your bushings out every year, when you annual your airplane. [You DO annual your ultralight, don’t you?])

The electrical harness came with the engine. We bolted the control box below our existing panel on the left hand side.

We didn’t use the battery which Polini had packaged. Being Lead-Acid, it was too heavy and archaic for such a svelte airplane or engine. We upgraded to a Lithium starter battery, packing more cranking amps and less weight.

I wanted a conventional throttle (push to apply power), so I engineered a reversing connection on the throttle cable. Push to increase power; pull to go back to idle.

The propeller we chose was a Powerfin 3 blade, “B” type with machined aluminum hub. The diameter is 54″ on this beautiful propeller. It has proven to be a perfect companion to the Polini.

The radiator was mounted below the engine, in the airstream. Polini supplied the radiator, and first flights proved it was effective, in fact, way too effective. I was unable to get water temperatures over about 120 degrees fahrenheit, which is below their
mininum recommended operating temperature. So we downsized the radiator to a tight little motorcycle radiator which I’d found on Amazon. We just used one of the pair; the item is a left and a right. With some welding, both would be usable, but the side without a cap was usable out of the box.


The radiator set may be purchased here: http://www.amazon.com/dp/B00B150O9A/ref=pe_385040_30332190_TE_3p_dp_1

Another important detail, unique to this engine, was the ability to monitor the water temperature. I designed a water temperature gauge combined with an EGT gauge, and came up with a new Belite instrument. The water temp gauge is matched to the unique probe which Polini uses, and I carefully calibrated it using a boiling pot of water in my kitchen. Not many guys get to drop a probe in the hot water, then take electrical readings off the voltmeter, all with the approval of significant others. Thanks, Kathy!

The panel in our ProCub included this new instrument. You can see it in the photo below.

water gauge panel

This instrument also spits out an alarm signal, so that any over-temp on the water temperature may be used to trigger an alarm light, such as that found on our Multi-Function Instrument. (The MFI has a row of three indicator alarms on the top; another is used for a low fuel indication.)

In nearly 50 hours of operation, I’ve only seen the temperature gauge climb off the top of the scale once. That was after we’d installed the new radiator, and hadn’t bled air bubbles out of the water pump after the coolant fill. Since water wasn’t moving, the engine water temperature quickly shot off the top side. I shut the engine down, and the problem was quickly diagnosed.

Speaking of coolant, we’re using 50/50 mix, and the engine takes about a half gallon of coolant to fill. That’s four pounds of water.

The weight of the engine including radiator, hoses, exhaust, etc., ready to mount, is about 54 pounds. This doesn’t include coolant.

Polini’s technical specs on this engine are here:


It’s time to take the Polini Thor 250 flying. Here’s the observations.

After filling the tank with 50:1 premix gas, engine start is easy. I’ve gotten in the habit of giving a few squirts on the miniature primer bulb (included from Polini!) until a few drops of gas drip out the overflow line. Hit the starter; the engine is running.

At low idle, the propeller may not engage — it has a centrifugal clutch. Very unusual.

The first observation is that the vibration level is very low — almost GA standards low. I’ve compared it to the vibration level in a four cylinder airplane — it’s that low. It’s a mind-blower if you’re expecting the vibration that competitive single cylinder or even twin cylinder engines provide.

With application of power, acceleration is best described as brisk, and our Belite ProCub is soon off the ground and climbing out a fun angle, even at gross weight.

With the improved radiator, temperatures have never exceeded 155 degrees, even in full power climb. They’ve generally run around 135 or 140 degrees.

Did I mention the smoothness? The engine gives no vibration to the airframe in a ProCub while in cruise.

Engine RPMS peak at around 7800 RPM (and Polini recommends 7500 RPM as red line). At full power, the engine makes the ProCub scoot — in experimental configuration, the airplane will move along at 75mph IAS. FAR Part 103 prohibits setting the prop angle so that this is achievable. In other words, keeping increasing propeller pitch on the ground until RPMs drop and speed drops. Cruising is comfortable anywhere between 6200 RPM and full power. Loitering is possible at 6000 or perhaps even 5800 RPM, depending on your weight, airframe drag and propeller pitch settings.

Fuel consumption at low cruise is less than 2.1 gallons per hour — setting a new standard for 2 stroke engines. Our ProCub can fly with the fuel sipping Polini and 5 gallons of gas for nearly 2 1/2 hours. Sorry, fuel consumption at full throttle has not been determined.

I’ve found the engine very comfortable to fly behind. The proof of this was my 1000 kilometer (620 miles) flight from Wichita to Oshkosh earlier this year. A video flight report from that trip was published here:

The smoothness of the engine is amazing (that’s at least the third time I’ve said that; I won’t bring it up again) and I like the power of the engine as well. I’ve flown my ProCub in some rough air; the engine powered me out of situations that would have been scary for a lower horsepower engine. Sometimes, there’s no substitute for power.

I’ve run the engine mostly on 91 octane auto gas (with the 50:1 mix, of course) and I’ve also run a couple tanks of 100LL. It can be done, but it makes me nervous long term because of plug fouling issues and the small gaps in the plugs. Polini has indicated that the use of up to 10% ethanol is OK. Just make sure the octane rating is correct.

All in all, the engine has provided the best flight experience I’ve had in a Belite.

Now, let’s talk honestly about the problems. There have been five problems with the engine, none of which were really the fault of the engine…

1) We blew the seams on the welded muffler which Polini provided.
2) An ignition coil failed. That’s a good reason to only fly behind airplanes with dual ignition.
3) An ignition electrical connection failed. Same as above.
4) The starter motor failed.
5) We found some bad soldering in the electrical box which Polini supplied.

The blown muffler was dramatic in flight but a non-event in reality. I was in cruise; the engine sound suddenly changed to obscenely wrong; RPM dropped 3 to 4 hundred; I looked around for a suitable field for a precautionary landing. Since the engine was still running, I decided to limp to home field, with a continuous eye for the off-field landing. Given the evidence in flight, it strongly suggested a muffler problem, and that is what it was.

This got Polini’s attention. They responded by redesigning and shipping a new muffler to Belite. We’ve never even installed it — we repaired the existing muffler to a higher standard, rewelding practically every seam, and it’s never given us any more problems. But I’m thankful that Polini improved the muffler for the next customer.

The failure of the ignition coil seemed to be random badness. The coil had a short from the step up side over to the primary. It’s a common coil from a well known manufacturer, and I know their products are in other engines in the ultralight market.

The electrical connector was one of several ‘push-on’ connections that got some corrosion in it. Easily fixed.

The starter motor was more random badness, but I’ve been wondering if it isn’t because we’re using a hot lithium battery for starting the engine. The copper braids to the commutator brushes broke. In any case, the starter motor is readily available everywhere and is used on many different products, such as snow machines, ATVs and so forth. You can even get the starter at NAPA!

We found some bad soldering in the electrical box. This was an irritant to my 35 years of electrical design and management experience, but I’m very confident Polini will never ship a marginal soldering connection to us, after I sent them a carefully worded email with pictorial evidence of the bad soldering. We found the soldering issues while changing the box from push-button ignition test to more conventional aircraft toggle switches.

So all the problems boil down to a bad muffler, subsequently redesigned, and some electrical unhappiness which has been resolved. General engine operation through the last 47:30 of use has been just awesome.

So is this the best ultralight engine ever? Well, I haven’t flown behind everything, and I’d still love to fly behind a wankel someday… but it is clearly the best engine we’ve found for our aircraft.

Caveat: Belite is a dealer for Polini.

Terry Strip 1

22 thoughts on “Polini Thor 250 Engine Review — The best ultralight engine we’ve seen

  1. Thanks for the review, James. I’ve been scouring the Interwebs for months looking for precisely the information you presented, and unfortunately there just isn’t much useful, real-world Polini info out there.

    It had occurred to me that perhaps the Thor 250 Dual Spark might just be the Rotax 447 replacement that Part 103 manufacturers and pilots have been looking for. The Polini puts out slightly less HP (about 3HP shy of the 447), but also weighs significantly less. (Rotax lists the all-in weight of the 447 as being between 82-100lbs, depending on the chosen gearbox). That’s a 24-42 lb. difference…not insignificant in the 103 arena. And then you still have the added bonus of the Thor 250 being a more modern design than the 447, with all the features you mentioned.

    I’m a little surprised more U.S. Part 103 manufacturers and dealers haven’t offered the Polini line for their 28-40HP engine needs. I only know of Northwing offering a Polini on their Solairus. It seems like Aerolite, Quicksilver, Kolb, CGS Hawk, Sky Cycle, etc. could put the 29-36.5HP Polini Thor models to good use. If Belite, Air Creation and Northwing can do it (and I’d say all three have done a splendid job implementing the Polinis), perhaps these other manufacturers can as well. Then again, this often tends to be a conservative industry, where companies often seem reluctant (somewhat understandably) to take a chance on a new engine.

    Speaking of new engine options, have you ever looked into the 33-38HP Briggs and Stratton engines for Belite, such as those used on the SiroccoNG and Bautek trike? Those engines seem like they have some potential too (especially with regard to reliability and low fuel consumption), although I understand they don’t necessarily produce a ton of thrust and may also be pushing reasonable weight limits in the Part 103 space.

    Thanks again for this review. I love how Belite keeps experimenting and innovating! It’s refreshing to see, especially in an area of light aviation where innovation sometimes seems elusive.

  2. James,
    Great article about the Polini engine. With a bit of luck, lots of self-discipline and the right amount of time my Belite Ultracub may get close to the “time of an engine” state this winter or early spring. I have been slowed down by a 4 week trip and the loss (stolen) of about $3,000 worth of needed hand/power tools for building the airplane – but they did not steal any part of the airplane! I am slowly replacing the tools which allows me to do some work each day.

    Question: I assume it will, but how well will the Polini work with the Ultracub? My bet is that it will, so I may be asking you for some more detailed installation instructions.

    Really enjoy your blogs.

    Steve H.
    Amado, AZ

  3. James, thanks for the review.

    I’m happy that you’ve noted that great engine balance is important–I surely agree, given the typical buzzing speeds of two-cycles.

    One thing you did not mention in your “wish list” engine preferences: I was thinking that the overall aerodynamic profile (where and how big is that hanging muffler, for instance) should be a significant consideration in aircraft.

    Yes, I’d like to fly a Wankel sometime, too. They sound great (fly-by sounds are available at aircraft conversion sites) but have two major drawbacks:
    I. the fuel burn is astronomical
    II. three flames out the exhaust for every single tumble of the rotor (out of a single pipe) assures extreme heat, compared to reciprocating 2 or 4 cycle engines.

  4. James,
    Thanks so much for your write-up and constant experimenting to improve your products! I have to admit I was baffled at first, since I’d been searching for a respectable ultralight manufacturer that had found a way to make a four-stroke fit within the weight envelope of the ultralight regulations. I was so pleased to find you’d done just that, only to find out I was reading an article from before your revelation with the Polini.

    As a bit of background, I’ve been a private pilot since my freshman year in college, but took up foot-launched “powered paragliding”, or “paramotoring” as an affordable way to continue to get off the ground while putting my children through college. Interestingly enough, a smaller Polini Thor (air-cooled) was on the final short list for my paramotor frame but I ultimately went with a Polish-made “Ventor” 25 hp engine. I have to admit that as much as I love this “raw” form of flying, I’ve never grown fully trusting of my 2-stroke that can actually peak out close to 10,000 rpm! (What can hold together long at those rotational speeds?)

    Now as I reach 54, and my “children” are approaching financial independence of a sort, I am looking for that reliable, quiet, smooth, efficient short field little plane that will keep me from worrying some minor medical issue will ground me from its use. I could just downgrade to a sport pilot and find one of those planes that meet my objectives, but a) my wife no longer has the stomach to fly with me, b) the cost differential is fairly significant, and c) I’ve grown fond of the simplicity of light flight.

    Your article does provides a nice comparison of pros/cons but a few other pros for the 1/2 VW come to mind:
    1. Just the sound of the lower-reving 4 stroke
    2. The assumed longer TBO time of the 4 stroke
    3. The convenience of not needing to mix gas/oil

    But offsetting, perhaps:
    1. Your repeated testimony as to the smoothness of this particular Polini
    2. The additional low-end torque providing shorter take-off’s / better climb rates
    2. The weight savings that seems to allow for electric start from within the cabin…I’m not at all a fan of hand-prop-starting an engine.

    So I am down to reliability and TBO, which, as noted by a previous writer, there does not seem to be much information available online on this Polini Thor in this regard. Perhaps the TBO times are much longer than I assumed due to the more constant engine temps afforded by the water cooling?

    So my lengthy writing seems to have boiled down to this one question:

    Do you have comparative data on TBO for both the 1/2 VW and the water-cooled Polini Thor?

    Thanks for your attention, if you made it all the way to the end of my musings….

    Current short list: Belite Ultracub, Kolb Firefly, Backyard Flyer (swing-wing with Generac 4 Stroke)

  5. Pingback: POLINI’S THOR 250 DUAL SPARK

  6. Great read. Love the plane. Love the engine. Happy with the Cub yellow. Suggestion, the plane would look great in Olive Drab matte and black-white-black-white-black D day markings.

  7. Mr UltraCub, & Mr ProCub, I believe your Blogs on the Belites are the best info I’ve ever had that covers every question I can thank of. We, your audiences, really thank you, in giving us the info needed to survive with our rides. Again, thank you!!!

  8. Great article.
    I have an Standard Xair with a dire need of weight loss.
    The Polini 250 duel spark definitely looks like a good replacement.
    What are your thoughts. .?
    Once again
    Thank you for a great article

  9. Thanks for the nice write-up.

    How does the clutch work? I understand that you can start the engine without the prop, and engage as you increase rpm. But what happens when you get an engine failure, can the prop disconnect from the engine, run away and overspeed?

    Have you done any more fuel-consumption calculations? Oh, and is a smooth engine…? 😉


  10. Howdy James, interested in this engine, how many hours have you put on it now? Is there a maintenance schedule? TBO? Do you know what the shipping cost average? Thanks!

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