Sierra Rotor/Shaft Assembly 18-99308 for 4/6 Cylinder Outboard Engines

The Sierra Rotor/Shaft Assembly, part number 18-99308, is a replacement ignition rotor and shaft assembly built for 4 and 6 cylinder outboard engines. This component is a core part of the engine's ignition system, working with the distributor to route spark to the correct cylinder in firing order. As the engine's original rotor and shaft wear from heat cycling and continuous rotation, ignition timing accuracy and spark delivery can degrade, leading to misfires, hard starting, or rough idle. Replacing the rotor/shaft assembly restores proper spark distribution and helps the ignition system perform as designed.

This assembly is compatible with ignition systems from CDI Electronics, giving boat owners and marine technicians a straightforward option when servicing 4 or 6 cylinder outboard ignition components. It is sized and configured specifically for 4/6 cylinder applications, so it should be matched to the correct cylinder count before ordering.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-99308
• Product Type: Rotor/Shaft Assembly
• Cylinder Configuration: 4/6 Cylinder
• Compatible Ignition System: CDI Electronics
• Weight: 0.9 pounds
• UPC: 736983857217
• Category: Mercury Outboard Parts

Fitment

Fits 4 and 6 cylinder outboard engines.

Frequently Asked Questions

What engines does the Sierra 18-99308 rotor/shaft assembly fit? The Sierra 18-99308 is built for 4 and 6 cylinder outboard engines that use a distributor-based ignition system with a rotor and shaft.

What ignition system is the 18-99308 rotor/shaft assembly compatible with? This rotor/shaft assembly is compatible with CDI Electronics ignition systems used on 4/6 cylinder outboard engines.

How much does part number 18-99308 weigh? The Sierra 18-99308 rotor/shaft assembly weighs 0.9 pounds.

What does the rotor/shaft assembly do in the ignition system? It rotates within the distributor to direct spark from the ignition coil to each spark plug in the correct firing order, so accurate rotor and shaft function is essential for proper engine timing.

What are the signs that a rotor/shaft assembly needs to be replaced? Misfires, hard starting, rough idle, or inconsistent spark delivery can indicate a worn rotor and shaft, which is when a replacement like the Sierra 18-99308 is typically needed.

Is the 18-99308 a genuine Sierra part or an aftermarket copy? Part number 18-99308 is a Sierra branded rotor/shaft assembly, sold under the Sierra part number as listed.

Does the 18-99308 rotor/shaft assembly include the shaft as one piece with the rotor? Yes, the Sierra 18-99308 is a combined rotor/shaft assembly, meaning the rotor and shaft are supplied together as a single unit.

Can this rotor/shaft assembly be used on both 4 cylinder and 6 cylinder outboards? Yes, the Sierra 18-99308 is designed for use on 4 and 6 cylinder outboard engine applications.

What category of outboard parts does the 18-99308 belong to? The Sierra 18-99308 rotor/shaft assembly falls under Mercury outboard parts.

What is the UPC for the Sierra 18-99308 rotor/shaft assembly? The UPC for the Sierra 18-99308 rotor/shaft assembly is 736983857217.