OS Engines FSa-56 Four Stroke Powerhouse Review - RC Groups

OS Engines FSa-56 Four Stroke Powerhouse Review

Mike McDougall puts O.S. Engines' new 56 Four Stroke to the test!



O.S. has been a leader in the field of four stroke engine design for over 20 years, so it comes as no surprise then that they should be the first manufacturer to the marketplace with the next generation of four stroke technology. The FSa-56 Four Stroke engine has NO crankcase vent! It's sized as a exact drop-in replacement for the familiar FS-52 Surpass engine and the new design F-4040 quite muffler is compact and offers an infinitely adjustable tailpiece for improved flexibility of cowl installations. Let's get this baby fired up!

Displacement:0.569cu in (9.32cc)
Bore:0.945 " (24.0mm)
Stroke:0.811" (20.6mm)
Practicle RPM:2,400-13,000 RPM
Power Output:1.0 ps @ 10,000 RPM
Weight:14.8oz (419g)
Weight w/Muffler:16.26 oz (461 g)
Recommended Props:12x6-8, 13x6-7
Manufacturer:O.S. Engines
Available From:Great Planes Model Distributors and Fine RC Hobby Shops
Retail Price:$249.99


The new engine comes well protected with each part individually bagged and a hard air cell wrap encircling all.

Package Includes:

  • O.S. Max FSa-56 Four Stroke Engine
  • F-4040 muffler
  • Type "F" header pipe
  • Glow plug
  • Instruction manual
  • Two year warranty


  • Fuel: 5-20% nitro with 18% lubricant content
  • Propeller: Sport & aerobatic; 13x6, 12x6-8, Scale; 13x6-7, 12x6-8
  • Electric starter, glow starter, and field equipment

Product Highlights

A big performance boost for any 40-size plane!

Opening a new era in O.S. 4-stroke technology, the FSa-56 is the first in the new "Ventless" Crankcase Series of engines. The 56 has the latest advancements for boosting performance, and it mounts with no-mod ease in the same space as the FS-52.

The F-4040 muffler is new also. In addition to less noise, it features a multi-positional tailpiece with a rubber O-ring that helps prevent oil leakage.

A newly designed lubrication system eliminates the need for crankcase ventilation.

Since the new design doesn’t have a crankcase vent, what happens to all the oil that accumulates in the engine? Let’s follow it as it makes its way out of the engine. First the oil is pushed into the center of the hollow crankshaft helped along by the pressure of the piston on the down stroke during the power stroke and the intake stroke. It then exits the crankshaft through a hole located in front of the rear crankshaft bearing and just behind the camshaft gears.

After lubricating the camshaft gears and the crankshaft bearings, the oil continues up the pushrod tubes into the valve gallery. In the gallery, the oil lubricates the lifters and the valves.

This is where the magic takes place. The oil is drawn into a small hole that exits into the space behind the intake valve.

On the next intake stroke, the oil is drawn into the combustion chamber along with the fresh air and fuel charge. Pretty neat! Not only is the oil “recycled” until it is burned, but it is pumped along through the camshaft gears, and into the valve gallery to provide improved lubrication to these normally oil starved areas. Way to go OS Engines! Look for this new technology to appear in many of their new four stroke offerings.

The 40NA carburetor is equipped with a venturi that reduces the chance of fuel leaking out into the cowl, while also creating more positive air/fuel flow

The FSa-56 engine is now more tolerant of fuel lubrication oils. The manual states that the fuel should contain not less than 18% (volumetric) castor oil, a top quality synthetic oil, or a combination of both. This should allow the user a lot more flexibility in fuel selection.

Other Improvements include:

  • Stronger connecting rod, wristpin, and valve assembly
  • Decreased width camshaft housing for less interference with sides of aircraft nose section
  • Two Needle 40NA carburetor


The O.S. manual stated that only a short and simple break-in was needed and that the proceedure could be carried out with the engine installed in the model. The process called for running the engine with the throttle set at the fully opened position, and regulating the engine R.P.M. by opening and closing the needle valve one full turn. This process was used with increasing run time at the leaner settings until two tanks of fuel had been used. The engine was then ready to fly at a slightly rich setting. The needle valve was then closed a little after each flight until the full power setting was achieved after 10 flights.

The FSa-56 started so easily and ran so strong, that I decided to use it in my Aerostar Trainer that was scheduled for Buddy Box duty at our annual community flight training event. During the event, the engine started every time with just a touch of the starter and ran stronger with each flight. At the end of the event, the 56 had logged over 2 hours of flight time without a single problem.

Prop Testing

With break-in completed, it was time for prop testing.


I used two different fuels for the prop testing. The break-in and initial prop testing were done with POWERMASTER 15% Helicopter All Synthetic Fuel. This fuel contained 20% All Synthetic Oil. The second set of tests were conducted using POWERMASTER 30% Helicopter (Low Viscosity Oil) fuel. This fuel contained 23% All Synthetic Oil.

Testing Conditions

  • Temperature 75 Degrees F
  • Barometric Pressure 29.9 in Hg
  • Dew Point 32 Degrees F
  • Elevation 873 Feet

Test Process

The needle valve was opened an additional 1/4 turn between each run. The engine was started and allowed to warm up at midrange R.P.M. The throttle was then opened all the way and the needle valve closed until the engine reached a peak RPM. The engine was adjusted till it would hold a peak R.P.M and then that value was recorded. The engine was then throttled back to an idle and the throttle closed until the minimum reliable idle was obtained. The engine was required to maintain that R.P.M. for 30 seconds and then the throttle was shoved wide open. If the engine did not accelerate smoothly to full speed, then a higher idle R.P.M. was tried. Once the idle R.P.M. was established, the value was recorded, the engine was shut off, the needle valve opened an additional 1/4 turn, and the prop was changed.

Test Results

15% POWERMASTER Helicopter Fuel
11x6 Master Airscrew 3 Series 12,240 2280
12x5 Master Airscrew K Series 11,250 2250
12x6 Master Airscrew K Series 11,070 2080
12x7 APC C2 Sport Series 9,930 2070
12x8 Master Airscrew K Series 9,660 2050
13x4W APC C2 Sport Series 10,270 2050
13x6 Master Airscrew K Series 10,060 2070
13x7 APC C2 Sport Series 8,970 2040
13x8 APC C2 Sport Series 8,580 2040

30% POWERMASTER Helicopter Fuel
11x6 Master Airscrew 3 Series NA
12x5 Master Airscrew K Series 11,640
12x6 Master Airscrew K Series 11,370
12x7 APC C2 Sport Series 10,230
12x8 Master Airscrew K Series 10,200
13x4W APC C2 Sport Series 10,860
13x6 Master Airscrew K Series 10,560
13x7 APC C2 Sport Series 9,510
13x8 APC C2 Sport Series 9,060

Test Observations

After some testing, it became apparent that a reliable idle speed for this engine was something around 2000 R.P.M. While you could get the engine to idle slower with some of the larger props and with the higher Nitro fuel, it really wasn't a practical "useable" idle speed. Thus the idle speed figures were omitted from the 30% Nitro chart.

The FSa-56 was a real pussycat when it came to starting and idling and a real TIGER when it came to turning up the speed.

I included the 30% Nitro Heli Fuel test because a large number of four stroke pilots have been reporting excellent results using these higher nitro content helicopter fuels. An interesting result of the testing was that the smaller props only gained an additional 300 R.P.M. but the larger props gained about 500 R.P.M. That level of performance gain on a big prop could make the difference between a scale model that flies, and one that really performs.

More Flying

I had a Brad Shepard Stephens Akro model with an O.S. FS-48 Surpass engine that flew OK on an 11x6 Master Airscrew K Series prop, but I had to fly it at full throttle (9800 R.P.M.) all the time and it was somewhat limited in vertical performance. The new FSa-56 bolted right in without any modifications. I didn't even have to enlarge any of the cowl cutouts. I checked out my prop test chart and settled on using a 13x6 Master Airscrew prop. I decided to try one of the new S-2 Series props I had on hand and the 15% Heli fuel. What a combination! The Akro went from a chore to fly to a gem. With the FSa-56 I got unlimited vertical and a dependable idle I could count on 100% of the time. Now I could fly long, low speed inverted passes as well as multiple snaps and spins and the engine was always rock steady. You still can't wipe the grin off of my face!

Is This For a Beginner?

Absolutely! This is one of the most user friendly four stroke I've ever run. It would be great for a beginner or even as a first four stroke for an intermediate pilot. Seasoned flyers will appreciate the ease of operation, reliability, and power.


The new O.S. FSa-56 is a real Powerhouse of an engine. I highly recommend this engine to anyone wanting to improve the performance of any 40 size airplane. Try an O.S. FSa-56, you won't be disappointed.


  • New ventless crankcase design keeps your plane cleaner
  • Carb Venturi keeps the inside of your cowl cleaner
  • Sleek new muffler design is very quite
  • Fits same mounts as the FS-52
  • Great idle performance
  • Lots of power for its size


  • Muffler O-Ring becomes brittle after a while and can break and leak when you readjust the tailpiece
Last edited by Angela H; Nov 05, 2007 at 09:02 PM..
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Nov 06, 2007, 10:17 AM
Registered User
wow! what a little power house nice review Mike, looking at this motor for 3-d bird around 4lbs. How was the bottom end grunt with the 13x6s2 prop. These # are as good as the os 70s2 and the 70 is much heavier. what about fuel consumption, I would assume it to be good being an os. thank you David well i check the rpms on my 70os s2 today with a master air screw k 13x6 12.5% cool power and came out with 11,500 rpms so the 70 is a good bit more powerful
Last edited by byrdbrane; Nov 13, 2007 at 01:08 AM.
Nov 06, 2007, 11:53 AM
Registered User
I really appreciate the prop chart for the engine.
Nov 06, 2007, 12:02 PM
We shall serve the Lord
kingsflyer's Avatar
Thanks, David.

I have a Morris-the-Knife that I have a Saito 72 on right now that I've been eyeing as the next perch for the O.S. 56. I should see an improvement in performance just based on the weight reduction. The Stephens Akro weighs 5 1/2 pounds and has unlimited vertical, so your 4 pound 3D should fly very well. You may want to consider the 13x4W for hovering. The 6 pitch prop may give you too high a top speed and you may get surface flutter.

As far as the "grunt" factor, the FSa-56 flew the Aerostar trainer all day at 1/2 to 2/3 throttle and never missed a beat. However, my heart missed a few as my students put the plane in some very unusual attitudes. I'd leave my TX throttle set at Max on the Master so that when I took control, I had plenty of power coming to help get me out of trouble. The 56 would rev right up and fly through the "High G" recoveries and settle back down to 1/2 throttle when I returned control to the student. Fuel consumption was about the same as with my Saito 56 and better than with my Magnum.

Remember to view the test criterion carefully that I listed for the Prop Tests. Those numbers are maximum sustainable R.P.M. values. I would back off of these numbers and richen up my engine slightly before I'd fly my plane. I expect the FSa-56 to easily exceed the posted numbers once airborne when the props unload.

Mike McD
Latest blog entry: LEDs on my T-28
Nov 06, 2007, 12:50 PM
orenda635's Avatar
Looks like a solid engine. Seems a little on the expensive side to me though. It's the same price as the FS-91.
Nov 06, 2007, 12:55 PM
We shall serve the Lord
kingsflyer's Avatar
I'm hoping they go on sale for Christmas!

Mike McD
Latest blog entry: LEDs on my T-28
Nov 06, 2007, 04:33 PM
Registered User
Originally Posted by kingsflyer
I'm hoping they go on sale for Christmas!

Mike McD
That would be nice but the good sales are not usually at Christmas time, I guess they figure that the people buying RC presents are going to buy them anyway.
Nov 06, 2007, 08:07 PM
Registered User

Great Review Mike

Great review Mike. Since we belong to the same club, I've seen this engine in action and it's everything you say it is. Two observations. For reasons I can't explain, the results on this engine are superior to those Chris Chianelli reported in his review in Model Airplane News. But I know yours are accurate; once or twice my tachometer was involved. Second, at a $250 street price, I think the cost is pretty steep. For my money, I'd spend another $5 and buy a Saito fa-82.
Nov 07, 2007, 07:42 AM
We shall serve the Lord
kingsflyer's Avatar
While I agree the Saito 82 is about the same price, it's a heavier (+1oz) and a physically much larger ( +3/8" wider & 1-3/4" longer) engine than the 56. The O.S. FSa-56 will easily fit in most 40 sized scale and sport models while the 82 probably won't.

I remember when the AX series of O.S. engines first came out, they were more expensive than the competition. But today, I'd rather have a 55AX and pay the few extra bucks than buy someone else's 46.

Like the old saying goes "You pays your money, You takes your choice". In the 40 sized four stroke class, I choose the O.S. FSa-56.

Mike McD
Latest blog entry: LEDs on my T-28
Nov 07, 2007, 10:14 AM
Registered User
Is OS going to bring these updates to their larger 4-strokes?
Nov 07, 2007, 02:16 PM
We shall serve the Lord
kingsflyer's Avatar
I can't speak officially for O.S., but I understand that these improvements will be incorporated in their new engine releases.
Mike McD
Latest blog entry: LEDs on my T-28
Nov 08, 2007, 06:38 PM
Nemo in a blender
Connexxion's Avatar
Saw a while ago this link to the new upcoming OS engines:


Nov 08, 2007, 06:55 PM
We shall serve the Lord
kingsflyer's Avatar
The next O.S. engine of this new type will be the FSa-81. Watch for Ian Bange's upcoming review of this engine here on RCGoups.
Mike McD
Latest blog entry: LEDs on my T-28
Nov 10, 2007, 08:00 PM
Registered User
Cherokee Flyer's Avatar
Has anyone else tried a Graupner 11-7 on this engine? I find it workes great on a Koas.

Dec 07, 2007, 05:53 PM
We shall serve the Lord
kingsflyer's Avatar
Update: In the Letters section of this month's Model Airplane News they mention that they reran the RPM numbers on their test of the OS 56a. Their new RPM numbers are a lot closer to mine.

You get the straight scoop here on RCGroups.

Mike McD
Latest blog entry: LEDs on my T-28

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