Model Engine Company of America
What Causes a Bent or Broken Connecting Rod
"I was just flying along normally and the rod just bent/broke for no reason!" -- Not Quite

First cause - Hydraulic Lock - This happens when there is liquid fuel in the cylinder and an electric starter is applied to the engine. Liquid will not compress like a fuel and air mixture so the only thing that can happen is the rod will bend as the piston is stopped prematurely at the top of the stroke.

The engine will then become stiff as the piston is not aligned with the cylinder bore. The engine could start at this point and you would not realize you bent the rod. You may also just bend the rod slightly and not even notice.

If the engine does start with a bent rod you may be able to run it for awhile but eventually it will either seize or the rod will break. Many times the engine will quit midair so you think "I was just flying normally and the engine stopped!".

The bent rod causes two things --

A misalignment of the rod on the crankpin so the rod is pressing against the rear cover. This rubbing causes friction thus heating the rod and allowing the lower bushing to spin. Next the rod breaks. See HOW TO FIX A BROKEN ROD

A misalignment of the piston in the cylinder bore causing lack of clearance on the top and bottom of the piston on opposite sides. Hence more friction and eventually galling or seizure.See SCORED PISTON




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