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OS Engines 120AX Glow Engine Review

Rich Noon takes an indepth look at O.S's newest innovational addition to their AX series...and falls in love with every aspect of it!

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Displacement: 1.218 cu in (19.96cc)
Bore:1.197" (30.4mm)
Stroke:1.083" (27.5mm)
Practical RPM: 1,800 - 9,500
Power Output: 3.1bhp/9000 RPM
Weight of engine: 22.82oz (647g) without muffler,extension or adapter
Weight of muffler: 7.074oz (219.2g) with 90° adapter installed
Weight of muffler: 7.907oz (245g) with extension installed
Propeller: 15x10-12 or 16x8-10
Manufacturer:O.S. Engines
Available Online From:Tower Hobbies
Available Locally From:Your Local Great Planes Dealer

O.S engines' latest addition to their AX series engines!

Model Application: Ideal for 3D, Precision and Sport flying
Mounting Application:Mounts in same space as the .91FX (actual mounting dimensions vary slightly, see SPECS)
Internal Construction:Steel piston ring and liner, aluminum piston
Muffler:E-5020 "Power Box" with extension and 90° adapters
Carburetor: 70D with rearward angled needle valve for safety
Glowplug: O.S. #8 supplied with engine, A5 (OSMG2693) can also be used
Warranty: Two-year, limited

First Look

OS’s new 1.20 AX came in the usual OS blue box with their quality packaging. As I unpacked the box I was impressed with the radically different look of this new engine. The angled head and slanted needle valve caught my eye immediately...a sleek, almost “racy” look. (Like the plane that looks fast just sitting on the flight line.) The finish on the engine and muffler shows OS is paying attention to detail.

As I unpacked the box I was amazed to see it came with an exhaust extension and an angled muffler mount. Included are three sets of mounting screws for the different muffler mounting options. (Note that you cannot use the straight extension along with the angled mount due to the screw lengths.)


The 1.20AX has a number of new and innovative changes from the FX series that should be pointed out.

Note: The engine is designed to be run with a spinner. Some thinner props will not mount without one due to the length of the crankshaft threading.


Engine Alone22.82oz (647gr.)22.6oz. (646gr.)
With Muffler29.89oz (850gr.)29.43oz. (838gr.)
With Straight Extension and Muffler31.33oz (892gr.)31.61oz. (900gr.)
With Angled Adapter and Muffler30.41oz (866gr.)30.98oz. (882gr.)


Advertised specifications:120AX 91FX160FX
Crankshaft thread size: 5/16-245/16-243/8-24
Distance Between Mounting Holes Same Side: 0.98" (25mm)0.98" (25mm)1.18" (30mm)
Distance Between Mounting Holes Opposite Side: 2.3" (58mm)2.04" (52mm)2.52" (64mm)
Distance Center of Crankcase to End of Crankshaft: 4.3" (109.5mm)NANA
Distance Center of Crankcase to Drive Hub: 2.7" (69.8mm)2.61" (66.5mm)3.15" (80mm)
Width Between Mounting Tabs: 2.6" (67mm)2.40" (61mm)2.91" (74mm)
Width of Crankcase Below Mounting Tabs: 1.8" (46.5)1.67" (42.6mm)2.02" (51.5mm)
Length from Backplate to End of Crankshaft: 5.5" (139.9mm)NANA
Length from Backplate to Drive Hub: 3.9" (100.2mm)3.77" (96mm)4.54" (115.5mm)
Height/Bottom of Crankcase to Top of Cylinder Head: 4.3" (109.2mm)4.05" (103mm)4.85" (123.2mm)
Height/Bottom of Mounting Tabs/Top of Cylinder Head: 3.4" (85.7mm)3.20" (81.5mm)3.80" (96.7mm)
Distance center to center of muffler mounting holes: 48mm(1.89")1.69" (43mm)1.65" (42mm)

First Run

I couldn't stand it any longer. Cabin fever was setting in and the engine was screaming to be run! I mounted the engine to my PSP test stand and installed the 15X6-10 Zinger prop I was using for break-in. I loaded up the van and headed to the test area.

Now bear in mind it was 26 degrees with about 5" of snow to trudge through. Is this the sign of an engine addict or what?

I mounted the engine to the mounting pole and filled the tank with Byron 15% fuel. I opened the needle valve three turns, primed the engine a few rotations and hooked up the glow driver. About two revolutions of the electric starter and it was running! I thought to myself, this is an amazing engine!

The manual provides very clear break-in instructions, allowing even the novice to safely break-in and tune the engine properly.

Per the break-in instructions, I ran it breaking from four stroking to two stroking for two 16 oz. tanks. It never so much as stumbled the entire time. Last tank I leaned it out a bit and put a tack on it. It was turning the 15X6-10 at 9000 rpm still on the rich side! Idling at around 2000 rpm reliably.

The transition from idle to wide-open throttle was very good. With a bit more running time and a tweak of the low-end screw, it should be perfect. I wouldn't hesitate to mount this engine and fly it at this point...keeping it a bit rich and following OS's recommendations for the first few flights. I'll be mounting and flying this engine as soon as the weather breaks!

I'm not sure what the wind chill factor is behind a 15X6-10 at 26 degrees, but it's cold! I'll be adding more data on different prop performance and engine temps as soon as the weather allows.

Second Run Session

The weather finally broke a bit and reached 36 degrees so decided to head out and try a few different props for comparison. Here's what I found:

RPM & Static Thrust

APC 15X109360213013.26 lbs
Zinger 15X6-109380210012.57 lbs
APC 16X88790176015.11 lbs.
APC 16X108280186013.44 lbs.
MA 16X69120185015.38 lbs.

I used a static thrust program that computes thrust based on prop size, air density and RPM.

Sound Levels


Readings taken at app. 6 feet with an APC 16X10 prop installed.

The new box style muffler really does a great job. Very quiet engine for it's size.


I am very impressed with this engine. The new innovations and styling are amazing. Considering it will fit it the space of a 90- size engine is a definite plus! Being a ringed engine, it should last for many years of flying. Also it's very serviceable if one ever did manage to wear it out. This is one powerful, well-designed engine. OS did an amazing job on design and function. I'd recommend it to anyone wanting 120-size power in a 90-size engine. In my book, “It’s a keeper”.



Test flight

I mounted the 1.20AX in a plane called a "Something Bigger". Basically, a Something Extra blown up to 1.20 size. This plane has been used as a test bed for several different engines. It weighs approx. 11 lbs. with a 70" wingspan.

It reached almost 50 degrees so I loaded up and headed to the field. It was very overcast and not the perfect day for flying or video. But it's been a long winter and this was the best flying day we had seen in a long time. After fueling and starting the engine it was just a matter of setting the top end. A quick range check, and we were off.

The engine idled beautifully and accelerated instantly as I taxied out to the runway. I lined up on the runway and gave her a little throttle. I was airborne in less than 30 feet at half throttle.

After climbing to a safe altitude and getting the plane trimmed, I pointed the nose up and nailed it. The engine didn't miss a beat and the plane was accelerating straight up at an amazing rate. I throttled back as it became a speck and let it idle all the way down. As I leveled out, I snapped the throttle wide open. Instant response! The engine didn't hesitate a bit.

I was having a ball putting the plane and engine through its paces. I did the usual "burn up the sky" maneuvers: Snap rolls, barrel rolls, inverted, loops, hovering, torque rolls, etc. The engine performed flawlessly through all the maneuvers with no sign of stuttering or stumbling. The engine transitioned from extended idle to wide-open throttle perfectly. (I might also note I used the same OS #8 glowplug throughout all the break-in and flights.) The engine just did what I asked of it with no complaints.


OS has developed an amazing engine in the 1.20AX. Very smooth and powerful with no bad habits I've observed. Combining its small size, lower weight and awesome power, it's definitely a winner in my book. The design changes make for not only a sleeker looking engine but also make it a very functional, user-friendly engine. It performed flawlessly and produced amazing power. I'd recommend it to anyone wanting 1.20-size power in a 90-size engine.

Beginners could break-in and setup this engine very easily following OS's instructions. An experienced pilot will appreciate the size, power and response.

The engine did everything I asked of it without any complaints. I wouldn't worry a bit about mounting this engine in any of my hangar queens and heading to the field. Like the rest of my OS engines, I plan to fly it for many years.

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Sep 18, 2006, 04:36 PM
Pro Bro # 2398
GassPasser's Avatar
thanks RichN for a great review, im a 4-stroke kinda guy, but im sold on this engine, it will defintly go in my new Maxair Velox rev II..thanks again !!!
Sep 19, 2006, 07:25 PM
Registered User
Thread OP
Thanks for the kind words.

It's a great engine and has ran like a top since day one.

It, in the 5 lb. Maxair Velox, should be a lot of fun to fly to say the least!
Let me know how it does.

Thanks again,
Oct 16, 2006, 02:11 PM
Registered User
hyper7's Avatar
How do you think it will perform in the GP Skybolt (kit) ?

Oct 19, 2006, 06:16 PM
Pro Bro # 2398
GassPasser's Avatar
gotta have order is in !
Nov 21, 2006, 10:44 PM
It's hot here!
babenson's Avatar
Hey Rich,

Great review. I am ordering the TT Sept Fury and just barely hit the "Add" button on the OS .91. Do you know the weight difference between the .91 (w/ Muffler) vs. the 1.20?

Also, do you think I will get more speed out of the Sept Fury with the 1.20 than the .91?

Awesome review.
Nov 22, 2006, 11:55 AM
Registered User
Thread OP
Hi Babenson,

Thanks for the kind words.

Just saw the Fury. What a gorgeous airplane!

It looks like the 1.20 is about 5.5 oz. heavier. But this will depend on the muffler combination or type of muffler you need. Or if you'll be using a pitts or tuned pipe.

I'm really not into speed, but think it would have to pull the plane faster than the .91. The increased horsepower and bigger prop should make a big difference. I'd start with a 16x8 or 16x10 and go from there.

I think you'll really enjoy the engine.

Maybe there's someone here that has tried the 1.20 in a racing type airplane?

Thanks again,

Originally Posted by babenson
Hey Rich,

Great review. I am ordering the TT Sept Fury and just barely hit the "Add" button on the OS .91. Do you know the weight difference between the .91 (w/ Muffler) vs. the 1.20?

Also, do you think I will get more speed out of the Sept Fury with the 1.20 than the .91?

Awesome review.
Jun 04, 2008, 05:05 PM
Registered User

17 x 6

I d'like to know if you have tested an APC 17 x 6 with this engine or if you have the plan to test it, I'd like to know the max RPM, Idle and static thrust.
Even that is not listed in the manual, the manual states that you can use a slightly bigger prop.than those listed. I consider the 17 x 6 to be in the range of operation.

Great review thanks.
Sep 01, 2008, 02:29 PM
Live and let Fly!!
Carlitos L.'s Avatar
Yestarday, my brother and I did the break in of one of these beatys... 4 tanks (24 oz no sure) and we took some readings...

APC 16x6 = 9,930 rpms!!!!

After the reading, and a little richening, it was maden at 9650 rpms...

Too much power for a Seagull Edge 60.... Unlimited vertical just at half throttle....

We are thinking of removing the baffle

Hope this was helpful...
See you
Jan 03, 2009, 11:56 AM
Registered User
My friend bought one of these engines when they first came out. Running a APC 17X6 we were getting between 9000 to 9200 rpm on Byron 10% nitro 20% oil.

The perfect engine for his Funtana 90.
Aug 28, 2009, 10:00 AM
Registered User
Originally Posted by Carlitos L.

We are thinking of removing the baffle

See you
The baffle adds to the power and makes the engine very quiet.
Jun 09, 2010, 12:36 PM
Registered User

about .95 ax.


I wonder, what is the differece between Os 1.20 AX and .95 AX?

It is obvious that 1.20AX has a little bit more power but if we need higher speeds(which I need) rather than torque, should we prefer .95AX which has a higher rpm.

Even if 1.20 AX is not a 4 stroke engine, it acts like one, right??

.95 AX also mounts in the same place where .91 FX can.( where all .75 and .61 engines can also be fitted).

Can you give comments on this.

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