MerCruiser Engine Couplers

If the coupler fails, the engine will run, but no power will be transferred to the propeller. The flatplate--for which it's named--is then bolted to the drive system using the bolts provided with it.

In form, it's a steel plate or even a short shaft, with holes drilled though one end so that it can be bolted to the flywheel of the boat's engine. Usually, once the coupler lets go, there is no halfway. This is a Mercruiser coupler. The coupler is found behind the flywheel--that is, away from the front of the engine--and ahead of the stern drive on your boat. The engine coupler links the engine to the stern drive on inboard/outboard engines. The second type is called a "flexplate" type of coupler, identified by a shaft that's bolted to the engine and a triangular plate called a yoke that may be removed if the stern drive uses a shaft-type attachment, rather than a a shaft-type connection between the engine and the drive system. The "flatplate" style of coupler resembles the flexplate coupler, except that its yoke cannot be removed. General Motors-based engines, such as Mercury Marine's Mercruiser, shares its engine block with the Chevrolet Corvette. The shaft-type connection mounts to the engine's drive shaft as the shaft penetrates the flywheel. How do you know if your engine coupler is bad? Usually, once the coupler lets go, there is no halfway. Even though it may resemble a round, square or even triangular plate, it still amounts to a drive shaft that connects the boat's power source to the propulsion system in the same way that a drive shaft connects a car's engine to the drive wheels. The coupler is made of aluminum and rubber, which isolates the vibration between the engine and the outdrive. It has six additional holes on the opposite end, or side, that allow you to bolt it to the stern drive. And you will smell burning rubber in the engine room. The shape of the coupler is dependent on what type of motor. The engine coupler connects the engine to the outdrive, which is outside of the boat. Most marine engine couplers look like a short, stocky shaft with a plate on each end, reinforcing the image of the drive shaft. There may still be a bit if friction left and if you take the engine up to normal cruising rpms, you will find the boat will just go slower, maybe not over about 8 knots.