Model Engine Company of America

Here's the probable reasons for a connecting rod breakage in a marine engine.
 
RUNNING THE ENGINE OUT OF THE WATER AT HIGH RPM’s OR FOR EXTENDED PERIODS OF TIME.
When the engine is not in the water there is no load on the prop so the engine can over rev. This will cause the connecting rod to break or stretch and break at a later date. Since there is no load on the engine it will run at a higher RPM’s with less fuel and in turn less oil, this will not supply the rod bearing enough oil and will result in breakage. Be careful. Only run your engine at low rpm’s while it is out of the water. Perform carburetor adjustments after running your engine in the water. The only way to obtain the correct carburetor settings is to have the correct load on the prop.

IF YOU FLIP YOUR BOAT OR CAUSE THE ENGINE TO INGEST WATER WHILE IT IS RUNNING.
This will cause the connecting rod to break or stretch and break at a later date. This is caused by the piston trying to compress the ingested water which will not compress in turn placing extreme loads on the connecting rod. This condition can also be caused by fuel flooding the engine and trying to spin it with an electric starter.

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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