O.S. Engines O.S. .75 AX





The current trend in engines is one that would make Tim Taylor grunt with pride: “More Power”. As pilots push their planes to ever increasing limits in aerobatic performance they want engines that deliver more power in smaller sizes. Recently many manufacturers have started offering engines that are just that – bigger engines in smaller sized packages. It’s easy to understand why this appeals to so many pilots out there – the ability to have the power of a larger engine without the penalty in weight and size of bigger engines. The latest offering from OS is one that is sure to make a lot of pilots sit up and take notice. Their newest engine on the market is the OS .75 AX ABL 2-stroke glow engine. OS states that it is “The new power source for 60 sized aircraft.” It is a .75 cu displacement engine that has the same mounting footprint as the earlier OS .61 FX engine.

A lot of the planes I fly are 60 sized aircraft because I like the ease of transport and fuel economy of flying this size of plane. So when I was asked if I would like to take a look at the new .75 AX it was a pretty easy decision for me to make. One of my “throw around” planes is a RCM plans built Kaos 60 with an OS .61 FX which has seen a lot of airtime over the years. Since I am very comfortable flying this plane I thought it would make the perfect test bed for this engine. Since the 75 AX is billed as a “60 sized” engine I wanted to use a plane with a 60 already in it. This would let me truly test that the 75 AX could be simply “swapped out” with the old 60. It would also let me make an accurate comparison with the power difference between the 60 and the 75 AX.

So with a hearty Tim Taylor “arrrr, arrrr, arrrr… More Power” grunt let’s take a closer look at this engine……..





  • Fits the same mount and bolt pattern as the 61FX.
  • The angular head design looks great and significantly increases surface area for better cooling.
  • Includes the new “Power Box” muffler, designed with more volume for increased power while still being very compact.
  • A diagonally-placed needle means no remote needle valve is needed.



Price: $194.97
Displacement: 0.75 cu in (12.3 cc)
Bore: 1.02 in (25.8 mm)
Stroke: 0.93 in (23.5 mm)
Practical rpm: 2,000-16,000
Output: 2.4 hp @ 15,000 rpm
Weight: 19.4 oz (550 g)
Includes: glow plug, E-3010 muffler
Recommended Props: 14×6, 14×8, 15×7






  • Mounting lugs are the same dimensions as a 61 FX
  • Larger head for more cooling
  • Rear angled needle valve
  • Easy starting
  • Reliable low idle
  • Pulling power




  • The engine is “thirstier” than it’s 60 sized counterpart.
  • Larger prop size may not fit on existing setups





The Engine



The OS 75 arrives packaged in a plain blue box that is so familiar to any OS user. Opening the box reveals that the engine and all parts are well packaged and protected against damage. The contents of the box are the engine, the Power Box muffler, an OS #8 glow plug, and a small plastic bag containing the muffler bolts and a needle valve extension. One word of note here. Make sure you look for the glow plug in the box. I found the plug at the very bottom of the box and didn’t actually find it until after mounting the engine on the plane. A first look at the engine will reveal that it is a bit different than most will be used to. OS has increased the size of the head and angled it forward in order to give the engine more surface area. This helps keep the engine cooler while running. Another change that will be noticed right away is the muffler. The muffler is now rectangular shaped as opposed to the old round mufflers on previous OS engines. This is the new Power Box design that OS has come out with. OS states that this muffler will help quiet the engine and still increase the power output. One other thing of note is the use of a jamb nut for mounting the propeller. Jamb nuts are usually found on 4-stroke engines, and are rarely seen on 2-strokes. But it is nice to see that OS has included this with this engine as it will help keep the prop on during flight.



One of the big “selling” points of the OS .75 AX is that it will mount in the same space as an OS .61 FX. Pictured above I have put the new .75 next to an older .61 FX. Sitting side by side with the .61 it’s easy to see that the lower part of the engine body is pretty much the same and that the mounting holes are indeed identical. That’s where the similarities end. Comparing the two engines the .75 stands out with a larger carburetor, as well as more head area to accommodate the larger piston. Also increased is the amount of cooling fins on the .75.

Another notable change with the new engine is the location and position of the needle valve. The .61 FX had a remote needle valve setup which placed the needle valve at the rear of the engine. The .75 AX has moved the needle valve back up to the carburetor, but has slanted the needle valve back away from the propeller. This allows for easy adjustment of the needle valve with the engine running while keeping delicate fingers back away from the spinning propeller. Also included is a length of braided steel wire and a small knurled knob. This is included to use an extension to the needle valve.



The Manual



OS has always provided excellent manuals for their engines, and the manual that comes with the .75 AX is no exception. The manual is a 4″ x 6″ 45 page document that is well written and easy to understand. After the mandatory safety warnings the manual starts off by explaining the basics of the engine, including talking about the various parts of the engine. The manual does a great job of explaining how to properly operate the OS 75 AX as well giving a complete break down of all the parts of the engine. Also included with the manual is a sheet of color decals that can be used to liven up the looks of your plane.




NOTE: When choosing a fuel for the OS 75 AX keep in mind that this is an ABL engine and the fuel should be suited as such. OS recommends that for non-ringed engines such as the 75 AX a fuel with some castor oil should be used, such as Morgan Fuels Omega blend fuel. Morgan fuels state that the Omega blend is specifically formulated for ABL/ABC engine applications. For engines such as the 75 AX the castor oil in the fuel provides extra protection from a lean engine run where a 100% synthetic lubrication would not provide as much protection



To break-in the OS 75 AX I mounted it in a test stand instead of in a plane. While setting up the engine to break-in I noticed something that is normally not seen on a 2-stroke engine; a double nut mounting system. Normally these are used on 4-stroke engines to keep the propeller from being thrown in the event of an engine backfire. I imagine with the extra power of the OS 75 they chose to use a double nut for the same reasons. I used a Master Airscrew 14×8 sport prop for the break-in, which is also the same prop that I used when the engine was mounted in the plane.


With the tank filled I opened the needle valve 2-1/2 turns as directed by the instruction manual. To prime the engine the throttle was opened fully, the carburetor was opened fully, and the engine hand cranked until fuel was pulled from the tank into the carb. With the throttle set back to idle position the glow driver was placed on the glow plug and the engine was flipped with a chicken stick. I was pleasantly surprised when the engine fired up on the second flip of the stick and idled smoothly. To break-in the engine the manual for the OS 75 AX calls for alternating the engine between a lean “2-stroke” setting for 10 seconds to a rich “4-stroke” setting for 10 seconds, and continuing this throughout the first tank of fuel. This procedure was followed for the first tank of fuel with nothing of interest to report. The engine alternated between those mixture settings with no problems. Although the manual says that the engine is ready for flight after the first tank of fuel I chose to go ahead and run a second tank of fuel through the engine in the same manner as the first. The only reason I did this was because I wanted to open the engine up and lean it out in order to get a good tach reading while the engine was still on the test stand.

After a second break-in tank it was time to see what the engine could do. I fueled it back up and restarted the engine. After getting warmed up I opened the throttle up and leaned it out to the point where it would be when flying the plane. Swinging a Master Airscrew 14×6 prop the engine tach’ed out at 9,100 rpm. With the high-speed end checked I dropped the throttle back to an idle setting and the 75 AX settled in and idled a very smooth 3,000 rpm. I let it idle at that setting for about a minute to get it “loaded up” and then quickly opened the throttle back to full. The engine quickly went to full throttle without missing a beat. Satisfied that the engine was going to be a reliable power source for my plane it was time to take if off of the test stand and get it in a plane to see what it could do in the air.

One major point of note that I noticed while breaking in the engine was the new configuration of the needle valve. Instead of a remote needle valve the 75 AX has the needle valve mounted on the carburetor, but the needle valve is angled back at a 45° angle. This allowed for adjusting the needle valve while keeping the hand away from the spinning propeller. I’ve never been a huge fan of the remote needle valve setups found in the FX line of engines, so this new layout is a huge hit in my book. I really like the way this engine is set up.








The plane I chose for the OS 75 AX was my Kaos 60 that was built from RCM plans. I chose to use this plane for two reasons. First, this is a 60 size plane which I had an OS 61 FX on. Since the 75 AX is advertised to have the same footprint as the 61 FX it should drop right into the same engine mount that is already on the plane. Second, I have a lot of airtime with this plane and I’m extremely comfortable flying it. Because of this I felt that I would be able to notice any differences in power and handling that the 75 had over the 61. For these reasons I felt that this would a great choice to evaluate the OS 75 AX.



Swapping out the 75 in the plane was really anti-climactic. Mounting it into the plane was really as simple as removing the mounting bolts from the 61, putting the 75 AX in place on the motor mount, and bolting it in place. The engine did indeed fit exactly into the existing engine mount, with the mounting bolts matching up exactly. The throttle control arm was also in exactly the same place, and I was able to hook up the throttle control arm with no adjustments at all. The only thing that did not match right up was the needle valve. With the 61 I had a cutout in the side of the plane at the rear of the engine. With the needle valve mounted on the carburetor on the 75 AX I needed to make a new cutout for it in the fuselage side. But that was it. I was able to remove the 61 FX and install the 75 AX in its place in about 20 minutes total time. It was a very simple task to perform.



With the engine in place I ran into the downside of the new engine. A couple of small issues presented themselves that had to be addressed, but nothing too difficult. First was the balance of the plane. The weight of the 75 AX is 3 oz. heavier than the 61 FX. Because of the weight difference I checked the balance of the plane with the new engine on it. The plane was nose heavy because of the new engine. To balance the plane required 2 oz. of stick on weights applied to the tail of the plane. The overall weight of the plane with the new engine and tail weights was now 6 pounds 12 oz. The second problem I had was the propeller. The old 61 FX used a 12×8 propeller, and for the new 75 I had a 14×8 to go in its place. This was a problem because of ground clearance. With the new prop I only had about ¾” of clearance between the prop and the ground. I didn’t make any changes to the plane at that time because I wasn’t sure if I would keep the engine on the plane or not. Since the plane had a tricycle gear on it I felt that the nose gear would protect the prop for the most part.

With the engine in place on the plane it was time to head to the flying field to see how it would perform.









At the field I couldn’t wait to get the OS 75 AX into the air to see how it performed. I fueled the plane and a couple flips of the chicken stick brought the engine to life. I ran it up to full throttle to make sure it was still tuned. Since it’s still a new engine I ran it rich enough to see a little bit of smoke in the exhaust. The normal checks of pointing the plane’s nose up to the sky and then down to the ground showed that the engine was running well and ready to fly. I taxied out and pointed the nose of the plane into the wind, started the flight timer, and then moved the throttle stick forward. With the throttle at about 3/4 of the stick travel the plane lifted off the ground and was quickly gaining airspeed. I pulled the throttle back to about ½ and circled the field twice to make sure the plane balance was indeed correct. When I was satisfied I decided it was time to see what she could do now. I leveled out across the field and punched the throttle forward. When the engine was opened up I pulled the elevator stick back and pointed the nose upward. With the plane climbing upwards there were two words on my lips at that time. “HOLY COW!!”

The new engine pulled the plane vertically with no problems at all and only stopped climbing when I cut the throttle back. It really was quite impressive the difference it made. So now that I knew what kind of power I had I decided to play with it a little bit. As I said earlier, I have lots of experience on my Kaos and it’s one of my favorite planes to fly because of the capabilities it has in the air (anybody that has ever flown a Kaos will know exactly what I am talking about here). But with the 75 AX on the nose of this bird it was like flying a new plane. All maneuvers were performed with more authority as the engine had no problems pulling the plane around the sky. In fact, I was able to perform one maneuver that was very difficult for me before – a vertical 8. With the 61 FX on the plane I barely had enough power to pull the outside loop at the top of this maneuver, but with the 75 on the plane it pulled through the top of this maneuver with no lack of power at all. It definitely left me with a huge smile on my face to see it pull it around the sky. With the playing done I came around on a landing approach. I intentionally left it in an idle for a long period during the landing approach in order to see what it would do if I needed to go around. As I came in I “waved off” the landing and powered up to go around. The engine responded by quickly moving to full throttle without missing a beat. With the timer running down I decided it was time to stop playing and bring the plane in. As I came in and touched down the heard the prop hit the runway as the nose touched down, so I guess that nosewheel didn’t protect it as much as I had hoped it would.

I flew 4 more flights after the first and I got a pretty good feel for the engine. There are a couple of downsides that I need to mention here. The first is fuel consumption. With my 61 FX 11-13 minute flights are normal, but with the 75 I had trouble making it to 11 minutes before I was out of fuel. I made a deadstick landing twice because I ran out of fuel in the air. So it’s quite obvious that the 75 is definitely more thirsty than the 61 FX was. The second problem is one that I already mentioned: prop strikes. I had more of them throughout the morning as I simply did not have enough clearance with the larger prop on it. Neither of these problems is difficult to overcome as a larger tank would easily fit in the fuselage of the plane, and larger wheels would fix the prop clearance issues. I mention these simply so those planning to upgrade to this engine can properly plan for it.

After 5 great flights with this new engine I packed up and left the field with a huge grin on my face. The OS 75 AX really made for some great flying. It made an already fun flying plane an absolute blast to have in the air. It’s definitely a keeper and will get plenty of flying time in the future.




Simply put the new OS 75 AX is a “stump puller.” The 75 AX is a powerhouse of an engine packed into a compact 60-ish size case. The OS 75 AX will add new life and power to an existing 60 sized plane, or it can make a new plane wow people as the engine pulls the plane around the sky. The OS 75 is easy to start, has a very reliable idle, and a top end that packs a lot of power. I would highly recommend this engine to both beginners as well as experienced pilots. I’ve got a feeling that the OS 75 AX will soon become the standard by which other engines in this size class are judged. OS certainly has a winner with this new engine. Whether you are trying to add some “zip” to an existing 60 sized plane, or powering a new 60 sized aerobat, the OS 75 AX should be the engine that you look at to take care of your needs!



OS Engines
Distributed Exclusively in the U.S.A. and Canada by:
Great Planes Model Distributors
P.O. Box 9021; Champaign, IL 61826-9021

Master Airscrew Propellers
Windsor Propeller Co., Inc.
P.O. Box 250
Rancho Cordova, CA 95741-0250
Website: http://www.masterairscrew.com/

Product used: Sport 14×8 propeller




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