OS .75 AX ABL Aircraft Engine with Muffler - RC Groups
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OS .75 AX ABL Aircraft Engine with Muffler

Ian Bange reviews the newest in the reliable line of OS engines for your .60 size aircraft, the OS.75 AX ABL.



Displacement:0.75 cu in (12.3cc)
Bore:1.02" (25.8mm)
Stroke:0.93" (23.5mm)
Practical RPM:2,000-16,000 r.p.m.
Power Output:2.4ps/15,000 r.p.m.
Weight:19.4 oz (My scale showed 26.8 oz suggesting that Tower's weight is without muffler)
Recommended props:14x6, 14x8, 15x7 (See below for more on this)
Manufacturer:OS Engines
Available From:Tower Hobbies
Retail Price:$194.97

OS has a long history of producing top quality RC engines for many applications. Fifteen years ago, my first trainer was powered by an OS engine, and I have run their products in airplanes, helicopters and even cars since then. OS engines have always been among my favorite RC powerplants, so I was very excited to receive my new 75AX for testing.

First Impressions

I opened the Great Planes box and was rewarded with the sight of a familiar blue OS box with typically excellent OS packaging and all the parts of a shiny new OS engine.


  • Same mounting dimensions as the O.S. Max .61FX
  • Bi-Metallic Liner process (uses a double layer of plating material rather *More precise fit between piston and sleeve for better compression and longer life.
  • Power Box muffler design provides more volume for increased power in a very compact shape
  • Head design offers increased surface area for better cooling
  • Ball bearing-supported crankshaft
  • 61A carburetor with diagonally placed needle valve eliminating remote needle valve placement


  • O.S. Max 75 AX ABL Aircraft Engine
  • E3010 muffler
  • Carburetor
  • #8 glow plug
  • illustrated instruction manual


  • Aircraft blend fuel containing 5%-20% Nitromethane with at least 18% lubricant

Inspecting a new engine is always fun, and the OS had some new surprises in store for me. This is my first encounter with the new AX series engines, and there are some pleasant changes from the old FX series.

The first and most obvious change is the new power box muffler. This muffler is worlds different from older OS mufflers and should yield interesting results. There is a new square shape that increases volume without being more difficult to fit in a cowl. There are larger attachment bolts and a nice notch on the exhaust stinger for zip tying on an exhaust deflector if desired.

The cylinder head is now sloped backwards and is slightly larger than equivalent FX heads. This new shape should help reduce damage in the event of a crash, and the larger size of the cooling fins hints at good heat dissipation. Forward of the new head design is the new OS 61A carburetor. This new carb does away with the remote high speed needle that the earlier FXs used in favor of slanting the high speed needle backwards at a 45 degree angle. It includes a four inch flexible extension for the needle. I like this change as it removes the throttle lag induced by the old remote needle without sacrificing safety.


I used an OMP Fusion 67" profile for my test-bed.

I began my testing of the 75 by following the break-in procedures outlined in the engine's manual. These procedures call for one tank of fuel to be ground run at full throttle alternating between rich 4-stroke operation and the cleaner, but still rich, 2-stroke operation. I installed a Xoar 14x6 propeller and headed to the field to get started. The OS came to life on its first flips with a chicken stick after a light priming (be sure to check for hydraulic lock from too much fuel if you choose to prime the engine by covering the carb otherwise you may damage your engine). It ran through its entire tank with no balking. This was one of the easiest engines to run through a break-in procedure that I have ever had the pleasure of dealing with.

The break-in continues in flight with three to four flights flown at a rich setting. These flights continued to show the OS 75AXís reliability, ease of working the needles as well as the ease of starting without an electric starter, but only time will tell how the engine would react as it broke in and the needles were leaned out.

Prop testing

Once I had completed the OS recommended break-in procedure and flown a few additional tanks through the engine on a rich setting just to be sure, it was time to see what the OS could really do. I started by backing the prop down to a 13x7 APC and tuning the needles in for maximum usable power. To me, this means leaning the engine in to peak, then backing the needles out till the engine loses about 200RPM. At this point the engine will run pointed straight up and should not have any problems during aerobatic flight.

The first round of tests were done using 10% Cooperís fuel.

13X7 APC11,000-11,100RPM
14x4W APC10,900-11,000RPM
14x6 Xoar9,700-9,800RPM
15x6 Xoar8,000-8,100RPM

According to Tower Hobbies, the recommended prop range for this engine goes up to 15x7. With this in mind, I did all of my flight testing with a 14x6 Xoar propeller that, according to Tower, was a mid-range prop for the engine. The performance was not what I expected, but these RPM figures shed new light on the issue. With over a 1000RPM drop from the 14x4 to the 14x6 prop, the engine is not capable of spinning a 14x6 to anywhere near the RPMs needed to get into the 75's power band. This suggests that Tower has actually posted an oversize suggested range of props! After testing I would suggest that the range would actually be something like: 12x8, 13x6, 13x7, 14x4.

Just to make sure that it wasn't the low nitro fuel that I typically run that was holding the 75 back, I fueled up with some 20% nitro which is OS's maximum suggested nitro percentage. I tested both the oversize 14x6 and the smaller 13x7 to see how both faired on the higher-nitro fuel.

13X7 APC11,300-11,400RPM
14x6 Xoar9,800-9,900RPM

While the 13x7 picked up a full 300 RPM on the higher nitro fuel, the 14x6 only gained 100 RPM. More importantly to me, while I could hear the 13x7 getting into the peak power band of the 75, the 14x6 never did reach peak power, even with the higher nitro fuel. Again, the 14x6 is still too large for the 75 to handle.

There is the possibility that the OS will still break in to yield greater power and eventually swing up to a 14x6 propeller, however I do not believe that it could ever get decent power out of a 15x7. That said, I do not believe that the 75 needs a larger prop in the first place as it puts out good numbers with the smaller sizes.

Photo Gallery


There are engines that are cheaper and there are engines that offer higher performance than the OS 75 AX, but none that offer its combination of price, ease-of-use, and reliability. The 75 is an easy running 2-stroke engine. It will run well on a wide range of fuel and will make for an excellent sport or scale powerplant for most aircraft weighing 10 pounds or under.

Last edited by Angela H; Aug 21, 2007 at 06:25 PM..
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Aug 21, 2007, 11:35 PM
Retired Army Aviator
Cyclic Hardover's Avatar
Better off performance and cost wise with a G90
Aug 23, 2007, 09:18 PM
margaret.roberts's Avatar
We find removing the head shims on ax/fx motors transforms the power output with no detrimental effects using 5-10% nitro.

Aug 24, 2007, 10:19 PM
Suspended Account
Originally Posted by margaret.roberts
We find removing the head shims on ax/fx motors transforms the power output with no detrimental effects using 5-10% nitro.


Transforms the power output to what?

OS is known for their very low compression ratio (they are considered smog engines by most performance enthusiasts). Anything you can do to increase the CR will help. If you re-machine the head for something in the order of 16:1 or more things really get lively. But at the current price a new OS makes for a very poor engine kit.

Keep it stock and enjoy the sport performance and ease of ownership (other than the price).

Friends don't let friends fly nickel,
Aug 24, 2007, 11:34 PM
Registered User
tailskid2's Avatar
I'm assuming the stated 16,000 RPM figure was with a 6 x 3?
Latest blog entry: Next!
Aug 26, 2007, 08:57 AM
Registered User
shandee92's Avatar
That engine looks really cool
Aug 26, 2007, 10:02 PM
Registered User
bob w's Avatar

to Konrad

It appears you dont care for O.S. engines all that much. What is your first choice for a good all around engine . Say, maybe for a 40 size fun fly or maybe 40 size sport plane or even a 60 size plane of the same? One that does not weigh a ton or cost an arm and a leg. I really enjoy reading your post and you really have some interesting input to different posts , However , I have not seen very much of what you like to fly or use as an everyday plane. I would really like to enjoy,,,, maybe just some of the performance that you allude to. Thanks for you help.

Bob W in Bartlett Tn.
Aug 26, 2007, 11:33 PM
Bigger is better
ibange's Avatar
I'm not who you asked, but I do have a lot of experience when it comes to nitro engines, so I'll answer that quetsion:

There are higher performance engines out there than OS, and there are cheaper engines. The OS is just a good mix of the two with good reliability thrown in.

For instance: In a 40 size fun fly you could choose a 46 or 55 AX from OS. Decent power, decent weight, decent price.

As an alternative you could go with a 62, 72, or 82 saito, any of which should offer less weight or better performance. The price tag on the other hand will run you about $100 more for that performance and torque.

Another alternative would be the webra 50. Higher performance, but also higher cost and a little tweakier than an OS.

So you see, no matter what you go for, you give and take. There is no having your cake and eating it too.
Aug 26, 2007, 11:46 PM
Suspended Account
You are correct in that I despise OS engines. This really is not accurate, what I despise is their marketing of themselves as High Performance engines. They blatantly are not. I point to their offering in the high performance part of the hobby. They state that their ducted fan, marine engines and the few racing engines that they produce arenít covered under a warranty because of the nature of the product. This means they know they donít hold up.

Their successes in the sport arena has squelched most other manufactures s from being able to offer a truly performance oriented product. This isnít the fault of OS but rather the modeling public not knowing how to tell a performance engine from a sport engine.

As a result of OSís and Great Painís very successful marketing OS can command a high price. I find that for another 10% in cost you can find engines that are more performance oriented and have much more potential for performance in the basic engine than OS. This is why I often ask; why pay competition prices for a sport engine?

The root of my distain is really aimed at us the modeling public. I canít blame OS and Great Pains for their success at marketing. OS for the most part do provide an adequate sport engine just donít look at them as a performance engine, they donít hold up\!

Do a search under my name and you will find a litany of reasons with detailed explanations as to the flaws I find with the OS products. You will also find a few accolades such as the OS Wankle.

I posted in this thread to try to discourage people from trying to extract more power from the reviewed engine, as it wonít hold up. I also give a little reason why the engine will respond to the modification given.

I'm not a sport flier. When I fly glow I fly F3D pylon and F3A Pattern. FYI Iím not very good at flying ether. I'd also rather build my own engines than buy engines that are crippled by market demands.

I will not list what I think is better in this thread, as this thread is about the OS 75 not my preferences.

Friends donít let friends fly nickel,
Aug 27, 2007, 12:03 AM
Registered User
bob w's Avatar

engines for the masses

I completely understand where you are coming from. Most of the major engine manufacturers build engines that can be used by the general public . That is where the money is. Demographics is the determining factor in what goes to market. I think the mufflers O.S. provides with their motors are strictly designed the way they are because that is what will fit in the box. ( Kind of joke around here) Most of the things we buy now are designed to be consumed and disposed of . I am in the HVAC buisness and planned obsolescence is all too apparent.

I have the O.S. 55 and 75 and with a good muffler they are well behaved engines. with some muscle too. My 75 on my pipe is turning a 15X4 Zinger at a little of 12K and throttle response is right now. I have over two gallons through it and it is getting better all the time. It hovers a 5.5lb plane at a quarter throttle. forever.

If you would, send me a pm with what kind of engine you like. You just kind of make me feel like I am missing out. If there is a better engine I would like to have one too!!!!!
Aug 27, 2007, 12:51 AM
S.A.D. member
ivanc's Avatar
If you're looking for a sport engine which is not overpriced, then you can choose among Magnum/ASP, Thunder Tiger, SuperTigre, GMS, Tower Hobbies, SK.

Aug 27, 2007, 05:07 PM
Bigger is better
ibange's Avatar
Remember though, you get what you pay for, even in a sport engine. The manufacturing quality of those cheaper engines is sometimes a little lower, and you can end up with a hit or miss depending on which engine you get. That is one thing I have to give OS is that I have yet to run an engine of theirs that wasn't rock solid, if not the highest performance available.
Sep 03, 2007, 01:47 AM
Registered User
My personal experience with OS vs others is:
An ST G-51, installed in a TF Contender - never could get it to run right, even with lots of help from other modelers. Transition was terrible, and I had numerous dead sticks. After one-too-many walks in the weeds, I broke down and bought an OS 46FX. It just, well, works. Perhaps my OS doesn't give doesn't make quite as much power as some other engines of the same size, but for the above-despised general sport flyer, I wouldn't trade it for any other engine of the same size, for any price!

I sold the ST-51 on eBay, with full disclosure of its problems. Maybe someone with more experience or skill than I can get it to run right...

- Tom
Sep 08, 2007, 12:08 AM
Steve Benesh

I tent to Agree with Tom (tshugart3)

I'm a kit builder, scratch builder, ARF builder, and weekend "sport flier". AM not interested in a performance engine that's tricky to set-up or a bargain basement engine that's just not reliable. For example, I just tossed a GMS 76, purchased new, into the garbage can after a year of putzing around with it and replaced it with the OS 75 which is proving to be a pretty good engine. The half dozen OS engines I've owned over the past 30 years have proven to me that they're a reasonably good blend of performance, reliability, and fair price.
Sep 08, 2007, 07:23 AM
Registered User
turk1's Avatar
Gentlemen,Its not true you get what you pay for everytime.Please check SK engines thread on RCUniverse and check SK prices.I have 4 of them and happy with them.Good workmanship,clean design.Good power for sport flying.Unbeatable pricing also.

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