Model Engine Company of America

Here's the probable reasons for a connecting rod breakage in an aero engine.
 
USING AN ELECTRIC STARTER -- Really be careful if you mount your engine inverted.

When a plane is sitting with fuel in the tank it is possible for fuel to siphon into the engine crankcase. Attempting to start the engine with excess fuel in the crankcase may cause the piston to hydraulic lock. This occurs when fluid is in the cylinder and the piston is forced up. Liquid will not compress like a fuel air mixture in turn placing extreme loads on the connecting rod.

Many times I see modelers hooking up the glow plug and bumping the engine with an electric starter. YOU MUST first turn the engine over by hand WITHOUT the glow plug lit. Several flips will allow you to determine if the engine is hydraulic locked or not.

If you determine that the compression is too high from this condition, remove the glow plug and fuel line, then open the carburetor barrel to wide open position and perform the following:

Hold the airplane with the engine pointed straight down and rotate the prop slowly. This will allow some of the excess fuel to run out.

Next flip the engine over by hand and then with the electric starter with the carb wide open, glow plug still removed and fuel line disconnected.. This will clear the excess fuel from the engine. PROTECT YOURSELF & EYES FROM FUEL THAT MAY SQUIRT OUT OF THE GLOW PLUG HOLE.

Now put the plug back in, close the throttle to 1/4 setting and start the engine as normal.

USE OF WRONG SIZE PROPELLER

This will cause the connecting rod to break or stretch and break at a later date. This is caused by too little or too much load on the connecting rod. Too little load requires less fuel/oil in the engine and allows your engine to rev higher with less oil thus overheating the rod bearing and causing the failure.

Too big a prop causes the engine to labor, the rod had to absorb the force of the combustion because it just can't turn that big prop fast enough. This usually results in the rod bearing wearing out and the wrist pin holes in the piston elongating.

See prop chart here Being slightly off big or small usually isn't a problem, being extremely off is.

DO NOT RUN YOUR ENGINE AT AN EXTREMELY RICH SETTING.
An ABC engine needs to come up to temperature and running it at an extremely rich setting for a period of time will cause premature wear on the piston as well as the rod to stretch and break. This is because the cylinder is too cold and the piston is forced into the taper and sticks and is then pulled back down with the connecting rod. The normal warm up period will not hurt the engine but trying to run it very rich will because of the accumulative effect of 15,000 rpm’s or 15,000 sticks and pulls of the rod per minute. The damage may not be apparent initially but the rod may break in the future seemingly for no apparent reason.


DO NOT TRY TO BREAK YOUR ENGINE IN BY SPINNING IT WITH AN ELECTRIC STARTER OR DRILL MOTOR.
This is the worst thing to do. You cause the same problems as running it too rich but even worse. By doing this the cylinder isn't even close to the correct temperature and there is no combustion to help the piston back down the stroke. So by doing this you are forcing the piston into the taper and then pulling it back down with the connecting rod. Connecting rods are designed to take force in one direction only and doing this will cause the rod to stretch and break as well as premature wear on the piston. The damage may not be apparent initially but the rod may break in the future seemingly for no apparent reason.
 

The above information is provided as a guide. Since MECOA/K&B has no way of determining the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage to person or property from the use of this information.
 

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