ABC engines should be broken in exactly as they will be flown, except for the high-speed mixture setting. In an ABC type, the cylinder's bore (diameter) tapers from a larger diameter at the bottom to a smaller diameter at the top. The piston has a constant diameter that is almost equal to the cylinder's diameter at its bottom.

As the piston travels upward, the bore becomes smaller until, at the top of its stroke, the piston is slightly larger than the cylinder's diameter. However, the piston and cylinder react to the heat generated when the engine runs by expanding differently; the cylinder expands more than the piston.

Since the piston is larger than the cylinder at the top in an ABC engine, break-in involves the cylinder's wearing away to become an exact fit to the piston when both parts are hot. But most ABC engines are built with the cylinder slightly too tight. Therefore, when the engine is first run and heats up, the cylinder remains too small. During the break-in, the cylinder loses material until it fits the piston exactly when hot.

How much wear occurs depends on the engine's rpm and propeller load. Using the same propeller for break-in and normal running ensures that the initial wear pattern will match the run pattern. The only difference is that the engine will be run slightly richer than normal during break-in for extra cooling and lubrication. ABC engines normally have short break-in periods averaging five to 10 flights.

Ringed engines do not need to turn the same rpm during break-in as during flight, but they do need to run cooler than normal. Thus ringed engines require a richer fuel mixture during initial flights. Using a propeller that is an inch less in diameter reduces the engine load, and heat generated, while allowing the engine to achieve enough rpm for break-in on the ground with a rich mixture. Ringed engines usually require more break-in time, averaging 15-20 flights.

Before running any engine, use common sense and take every precaution. The airplane must be immobile, the propeller must be tight, all obstacles must be cleared, do not smoke, and do it outside. Wear eye and ear protection, and never stand to the side in the propeller arc or make adjustments from in front of the engine. Do not reach around the spinning propeller to make needle-valve adjustments, remove the glow driver, or for any other purpose! Make all adjustments while standing in the rear of the engine. Please!

I have taken far too many friends to hospitals through the years, watched too many microsurgeries, and hoped far too many times that they could reattach nearly severed fingers not to warn anyone reading this to be careful. There is no reset button once that propeller hits you. This goes for any type of propeller turned by any type of engine or motor.

Break-in procedures for ringed engines vary by individuals, but consider the following. Open the high-speed needle valve a half turn more than the engine directions state. Have the throttle wide open and the model properly secured. Prime the engine by holding one finger over the venturi, hold the propeller securely, and rotate it counterclockwise until fuel moves through the fuel line and nearly into the carburetor. Do not have the glow driver attached.

Connect the glow driver, making sure that any wire is clear of the propeller arc, and start the engine. Remove the glow driver. The engine will run at full throttle, but at an extremely rich setting. If the engine falters, close the needle valve (while standing behind the engine) just enough to ensure a steady run. The engine should be spitting raw, unburnt fuel from the muffler and running roughly 2,000 rpm slower than normal. Run the engine this way for five minutes, and then shut it down and let it cool.

Repeat this procedure twice more. On the third run, let the engine run rich for two minutes, and then "lean" the mixture; turn the needle valve clockwise or close it until the engine sound changes from a low-pitched tone to an alternating low-pitched/high-pitched sound. Stop there and let it run for 30 seconds, return to the rich setting for two minutes, and then stop it again and let it cool.

Restart and then lean the mixture to achieve that alternating sound, and let it run there for one minute. Richen the mixture again (open the needle valve), but only to a half turn less than the initial rich setting. Now the engine speed should be approximately 1,500 rpm lower than normal.

After one minute of rich running, lean to the alternating sound point and run for one minute. Continue alternating the needle-valve settings for five more minutes. Stop and let the engine cool. Restart and set the needle valve to the alternating sound point. Run the engine at this point for three to five minutes. If the engine holds rpm and doesn't seem to slow down, it is ready to finish the break-in while flying. Install the flying propeller. Total ground time is usually 30 minutes.