What follows relates to the OS LA .15 engine. It is based on personal experience. Your results may vary. The information that follows is not necessarily applicable to other .15 size engines. The reason for this article is to provide a proven path to success to getting your OS LA .15 to run well, predictably and powerfully.

Opening the box.
When you first open the OS LA .15 box, you will find the engine, its carburetor, two muffler bolts, and the muffler. Before you run the engine, for the first time loosen then tighten all the bolts in the head and the back plate.

It would be better to replace the head bolts with 2.5mm X 10mm socket head bolts from McMaster Carr www.mcmaster.com because the OS cross point head bolts are JIS (Japanese Industrial Standard) heads and if you do not have a JIS cross point screw driver you are likely to strip the bolts head as you attempt to tighten it. The muffler bolts are also JIS and can be replaced with 2.5mm X 25 mm socket head bolts also from McMaster Carr or Micro Fasteners. You do not have to replace any bolts for performance but if you value reliability, you should replace them or purchase JIS screwdrivers.

The OS LA .15 back plate is made of a phenolic material and includes the needle valve. The needle valve is exceptional and the best we have found for the engine but it is prone to being broken off in a mid-air collision so we typically cut it off and mount it separately from the engine. The back plate works very well but the bolts (again JIS cross point screws) are prone to loosening. It is a good idea to remove the back plate and smear RTV Silicon (red) sealant around its surface forming a gasket. We also put a little RTV in the boltholes to help them stay secure so they do not back out.

Break in
You have probably heard that many fliers simply bolt their new engines into their airplanes, set them rich, and go fly. It works for them, but if you want the best performance and gain the most knowledge of your engine, you will put it on a break in stand and follow a disciplined break in procedure.

The OS LA .15 is an ABN engine with schneurle porting and a “pinch” fit between the piston and cylinder. Cool! But what does all that mean?

The piston is made of aluminum that is the “A.” The cylinder is made from Bronze, that is the “B” and it is coated with Nickel, which is the “N.” When heated aluminum, and bronze expand differently. Aluminum expands less than bronze. The nickel coating is hard so that the surface of the cylinder is harder than the piston and as two surfaces wear to each other the piston “seats” into the nickel-plated bronze sleeve. For the moment just, remember that the sleeve expands more than the piston.

ABN and ABC (C=chrome plating) engines have an interference fit between the piston and cylinder when cold. They also have a tapered cylinder that is narrower at the top of the cylinder than at the bottom. When cold the piston is a very tight “pinch” fit to the cylinder at the top of its stroke as it rotated by hand. There is a definite tight interference between the piston and cylinder at the top of the stroke.

When you run the engine it heats up and the cylinder expands. Remember that it is tapered with a smaller diameter at the top than the bottom. It expands at the top (where all the heat of combustion is) and the tight pinch fit is reduced. The piston also expands but it expands less than the cylinder so the tight fit of the cold engine is changed to a very fine fit when the engine is up to operating temperature.

OS engines are fitted looser from the factory than racing engines. This is done so that they do not require prolonged break in. They are a close fit, at operating temperatures from the factory.

So, what are we trying to achieve in the break in process? Several things. First, we want to relieve the stresses inherent in the cast case during manufacturing. Second, we want to get the best piston to cylinder fit when hot that is possible. Last, we want to make sure that the piston connecting rod moves freely on the crankshaft crank pin. We can achieve all three objectives with the right break in procedure. Let’s look at the process.

For break in you should use a prop that allows the engine to run without effort. Effort equals excess heat and that is something we want to avoid. Use a prop two inches in diameter less than what you plan to fly on your plane. A 6”X 3” prop is excellent for breaking in an OS LA .15. It can be made by cutting down an 8” prop and balancing it well.

Before we start the engine let’s talk about fuels. Many fliers believe that the more oil you run in your fuel the better it is for your engine. In the 60’s, the Fox engines required 29% castor oil to run their best. Your OS LA .15 is made from different materials than the Foxes and has a “happy” oil range and it is not 29%.

Experience has shown that the OS LA .15 runs best on fuels that have a maximum of 18% oil. We run oil that is one third castor and two thirds synthetic. Oil does not burn and increases the cylinder pressures. Higher oil is actually bad for your engine as it will cause it to get hot and will stress its internal components during the combustion stroke.

You should break in your engine on the highest content of nitro you ever expect to run. The higher the nitro the hotter your engine will run. The higher the heat, the greater the amount of expansion of the piston and cylinder of the engine. When your engine goes “over the hill” and has a looser fit a lower nitro fuel, which runs cooler and causes the components to expand less may be used and actually cause the performance to improve.

Start your engine and lean it to the point where it is running at 90% of peak rpm. Run it for thirty seconds then shut it off and let it cool to the touch. It will take about five minutes to cool off. What this does is heat the cylinder so it expands. This keeps the interference fit to its minimum preventing undesirable wear on either the piston or cylinder. It makes sure the crankpin to connecting rod fit is started and finally it relieves stresses in the cast cylinder by getting it hot and letting it cool. Repeat this process six times. After the short runs do four 3 minutes runs pinching the fuel line to get peak rpms once every thirty seconds.

After you have completed this process, you will have gained some knowledge of how your engine likes to be started and run. You can now bolt it onto your airplane and make your first flights. It is likely that your engine will not make full power till it has an hour of more of running time.

Setting the needle valve for flight.
If you are using a standard plastic draw clunk tank and have drilled out the muffler baffle you are in trouble. Drilling out the muffler reduces backpressure to the fuel tank. Drilling out the muffler increases the high rpm range but it also reduces the amount of pressure fed to the tank and really affects fuel draw.

OS LA .15s do not have the ability to draw fuel well from a clunk tank. Many fliers peak their engines on a full tank of fuel and find their engines going lean during the match. You may find the following advice unusual but it will work. Try it in practice.

Fill your tank to 1/3rd full. Crank your engine and set your needle for peak rpms then back it off one or two clicks. This assures that your needle is set correctly as the fuel tank empties in a combat match. If you set your engine to peak with a full tank, it will go lean during the match, usually in the first minute. With a “draw” fuel system, you need to set the needle when the tank is less than half full. This process will make the engine run rich during the first part of the match but finish strong at the end.

Some folks claim that engines “unload” in the air. I think they do speed up but I don’t think it is correct to say they unload. I believe that as they start flying the airflow and air pressure in front of the airplane increases. This increase in pressure and flow means that more air is going into the carburetor. A rich setting of more fuel than air on the ground therefore changes to a balanced flow of air and fuel once the plane is moving. This is another reason to set the engine slightly rich on the ground. As soon as it starts flying the air to fuel ratio changes to a leaner mixture. There is an exception to this and that is for engines that are force fed fuel with a pressure system.

Over the last two years many fliers have switched from “draw” tanks where fuel is sucked via venturi vacuum to the engine to systems that force fuel to the engine. These pressure systems are simply called “bladders” for short.

The reason to use a bladder is engine run consistency. The consistency comes from reducing the number of variables in a draw or clunk tank set up. A clunk tank has pressure changes as it empties, a bladder does not, the engine must use vacuum and slight pressure to draw fuel into the engine so the engine is working to suck its fuel, a bladder forces all the fuel an engine can take into it. The limiting factor with an engine running a bladder is how much air you can get into and out of the engine. Drilling out the muffler baffle and stinger on an engine with a bladder dramatically increases performance and has no downside; you do not need muffler pressure with a bladder. You will still need to set your needle one click rich on the ground as the engine will be able to induct more air once in flight and up to speed. Bladders will yield 500 or more instant rpms and much more consistent engine runs.

When your engine starts to run poorly:
In most cases, if your engine performance drops off you have a leak in the engine or you have lost the piston to cylinder fit. The more common problem is the back plate leaking air. A tightly sealed back plate is important because it forms part of the lower crankcase pressure chamber. When the engine is coming down on its stroke, it is compressing the air below it. This pressure forces the fuel to go into the bypass between the cylinder and case and into the combustion chamber. If the back plate is leaking then fuel will not be sent into the combustion chamber.

A bad or worn out piston to cylinder fit is likely the result of running the engine too hot, or too lean. When cold it is hard to tell how good the fit of the piston to cylinder really is because the parts should interfere when cold. If they do not then you are likely looking at replacing the piston and cylinder set. Often a worn engine will run better when it is run cooler. Lower nitro content in your fuel will make it run cooler and therefore faster. I have often seen folks with poor performing engines going to higher nitro content, therefore running their engines hotter, and not seeing any increase in performance. This happens because the gains in combustion energy are offset by the weakened piston to cylinder seal caused by higher parts expansion.

We hope this helps you get your OS LA .15 to perform reliably