Failure Analysis
125
Engine Overspeed
The MTD engine is designed for a maximum speed of 3600 rpm. When the governor is unable to control the
engine rpm, the engine can accelerate past the safe maximum speed.
When an engine runs beyond its designed speed, a few things happen:
1. As the piston moves up and down in the cylinder, it builds momentum. The higher the rpm’s the more momen-
tum produced by the pistons. As the momentum builds, the connecting rods will start to stretch. When the con-
necting rods stretch, they get weaker. Generally speaking this is at the narrowest part of the connecting rods.
On most engines that would be about an inch below the wrist pin, but on the MTD engine it is at the wrist pin.
The force on the connecting rod is greatest when the piston transitions from the upward stroke to the down-
ward stroke. Because of this, most overspeed connecting rod failures will occur with the piston at top dead
center.
When a connecting rod fails, the piston stops moving but the crankshaft is still moving. This will allow the bro-
ken connecting rod to get knocked around in the cylinder causing more damage to it. Usually the connecting
rod will be in several pieces after it breaks making it hard to find where the first failure was.
2. All engines have vibrations and are designed to handle those vibrations, but in overspeed the vibrations
change resonance. Parts that can not handle the new resonance will crack. This may result in parts flying off of
the engine which is an unsafe condition such as when a flywheel shatters pieces of it fly off of the engine.
The vibration can also lead to fasteners loosening up. Evidence of this could be elongated mounting holes.
The area around the mounting holes may be polished due to the two surfaces rubbing against each other.
3. When an engine overspeeds, the moving parts can not pull the oil in between them. This allows metal to metal
contact. Because of this, signs of inadequate lubrication will show.
4. When trying to diagnose an overspeed failure, look at all the pieces. Individually the lack of lubrication, piston
position and condition of the connection rod will usually indicate separate failures. Collectively they would indi
-
cate an overspeed failure.
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