[AntonioUSA]

...Trovato l'articolo.
L'articolo è di Lou Melancon che ho avuto il dispiacere di conscere, ha un ego grande come una casa, ... i pollici che si è no vanno bene per fare l'autostop... ma è un esperto di motori, uno di quelli che se li porta a letto...
la lettura è lunga e in inglese... divertitevi, è quasi una decina d'anni che ti combattari degli USA usano questa procedura per rodare i motori... e vi spiega perchè ed entra nei dettagli tecnici etc... divertitevi... questo si riferisce al 2,5cc OS ma si applica a tutti gli OS da usare in competizione.

""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 2.6mm 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.