Model Engine Company of America
So you broke your rod ----- Be sure to fix it right

Click Here to read about the cause for marine engines ---------- Click here to read about the cause on areo enignes.

This is typically how a rod will break and the tell tale signs of the cause. The customer that sent this to us stated he used his starter to try to break in his new piston/liner assembly. See ABC Break in

He stated that he oiled the engine with Marvel Mystery Oil and proceeded to spin the engine with no glow plug in an attempt to loosen the fit. See ABC Break in

However this break definitely occurred while the engine was running at a fairly high RPM. This can be seen by the indentation of the crankpin hitting the rod after the break. More evidence of overheating can been seen on the crank pin as it is discolored.


The bronze bushing at the far left is completely coated with aluminum oxide. This was caused from the bushing overheating and spinning in the rod. As you can see in the close up of the rod, there are spin marks. The aluminum expands at a greater rate than the aluminum so a firmly pressed in bushing becomes not so firmly pressed.

The bushing itself has been deformed to a point that there is about a 1/16" of clearance between it and the crankpin. This was probably caused because the crank pin was not polished after the prior broken rod.

When the bushing heats up it in turn heats the connecting rod. When the aluminum reaches elevated temperatures the strength diminishes.


When the tensile strength is lowered from heat the material breaks.

Another scenario can also occur. The bushing can heat up the rod and damage the heat treatment of the aluminum thus lowering the strength. So the rod may not break when the damage occurred, it happens when the rod is under extreme stress which the rod could have handled if it was in its original heat treated condition.

An another possibility is the bushing could have spun just enough to misalign the oil holes rendering them non functional. This lack of oil will cause the overheating to occur when the rod/bushing is stressed at high rpm... and here comes the break, for no apparent reason.

This crankpin shows all the signs of lack of oil and overheating. The pin is discolored from overheating and bronze and aluminum has transferred to its surface.

Before a new rod can be installed the pin must be polished with 280 grit Emery paper. Grab the pin firmly with the sandpaper and rotate the paper around the pin.

All the bronze and aluminum must be polished off the surface. The finish must be smooth. Care must be taken not to deform the steel pin itself. The pin has a case hardened surface and should resist any actual alteration from the polishing process. If you able to deform the steel portion of the pin it may have been annealed (lost its hardness) when it was overheated. If this is the case the crankshaft must be replaced.

If you can see any sign of bronze or aluminum on the crankpin you must continue polishing it with more emphasis on removing the bronze and aluminum, not altering the crankpin.

When you are finished you should have a smooth polished steel pin to fit your new connecting rod to.
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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