The question of buying a flywheel versus a flexplate is an important one. These two components serve vital functions and help marine engines run smoothly. That means telling the difference and knowing when they need to be replaced is critical.

What Is a Flywheel?

Flywheels in manual transmission boats are thicker than flexplates because they have to withstand greater amounts of friction. They provide a durable and stable surface for the clutch. The extra mass adds to the engine’s inertia as it rotates, so the boat is less likely to stall as it starts again after a stop.

What Is a Flexplate?

Flexplates appear in automatic transmission boats. They provide a linkage through which the power of the engine is transferred to the boat’s torque converter. Unlike flywheels, flexplates bend along a single axis. This serves to absorb, store and stabilize the engine power output in a spinning mass until it’s needed by the transmission and gearbox.

Flexplate vs. Flywheel: The Similarities

The main similarities between flywheels and flexplates include:

  1. Circular shape and ring gears that marry them to the engine’s starter.
  2. Mounted to the crankshaft and provide a link between the engine and the transmission.
  3. Serve to balance the boat’s engine while it’s running.

What’s the Difference Between a Flywheel and a Flexplate?

It’s a common misconception that flywheels and flexplates are interchangeable. They’re somewhat similar in appearance and serve similar functions. But knowing the flywheel and flexplate differences is vital if you’re servicing your marine engine and need replacement parts.

The critical difference is that flywheels are found on manual transmission boats, and flexplates are used in automatic transmission boats.

When You May Need New Parts

Keep these tips in mind when deciding whether flywheel or flexplate parts need replacing in your marine engine:

Flywheels: The signs that your flywheel may have reached the end of its life include gear slippage, a burnt smell from the clutch facings, vibrations while using the clutch and failure of the clutch to release when it should. If parts of the flywheel are damaged or missing, it may result in clutch chatter, or vibration as the clutch releases.

Flexplates: Flexplates can become permanently warped. When this happens, it can result in a high whine as the starter spins and grinding or clunking noises during idling or while the boat is in neutral. If the weight in the flexplate is unbalanced, it can result in a noticeable vibration in the driver’s seat at almost any speed.

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