Sierra Water Pocket Cover Assembly 18-3219, Replaces MerCruiser 42724A3, Alpha One/Gen II

The Sierra 18-3219 Water Pocket Cover Assembly is a direct-fit replacement for the drive shaft housing water pocket cover on MerCruiser Alpha One/Gen II sterndrives. This part is sold as a complete assembly rather than a bare cover, so it comes ready to install without hunting down individual hardware. The assembly bundles the water pocket cover itself along with its mounting screw, gasket, water pump housing bushing, and water tube, giving you every wear component in this section of the drive shaft housing in one package.

The water pocket cover directs raw water flow through the drive shaft housing on its way to the water pump and engine cooling system, so a cracked, corroded, or leaking cover can lead to cooling flow problems or water pump housing leaks. Because Sierra ships this as a full assembly, you are also replacing the gasket and bushing at the same time, which is the recommended practice whenever this cover is opened up, since these seal points are the most common source of a repeat leak if only the cover is swapped.

This part is cataloged under MerCruiser Alpha One/Gen II drive shaft housing components and cross-references to MerCruiser OEM number 42724A3. It is intended for use on Mercruiser I and Alpha I stern drives.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3219
• OEM Cross Reference: 42724A3
• Sold As: Assembly (includes water pocket cover, mounting screw, gasket, water pump housing bushing, and water tube)
• Product Dimensions: 0.39 x 0.39 x 0.39 inches
• Weight: 0.29 pounds
• Finish: Machined
• UPC: 808282243852
• Application: MerCruiser Alpha One/Gen II drive shaft housing

Fitment

• MerCruiser Alpha One/Gen II drive shaft housing components
• MerCruiser I and Alpha I stern drives

Replaces (OEM)

• 42724A3

Frequently Asked Questions

What does the Sierra 18-3219 water pocket cover assembly include? The 18-3219 assembly includes the water pocket cover, its mounting screw, a gasket, a water pump housing bushing, and a water tube, so every wear component in that section of the drive shaft housing is replaced in one package rather than sold as a bare cover alone.

What MerCruiser OEM number does part 18-3219 replace? Sierra 18-3219 cross-references to MerCruiser OEM number 42724A3.

What stern drives does the 18-3219 water pocket cover fit? This assembly is cataloged for MerCruiser Alpha One/Gen II drive shaft housings and fits Mercruiser I and Alpha I stern drives.

Is 18-3219 the same as buying just a water tube? No, 18-3219 is the water pocket cover assembly for the drive shaft housing; the water tube is one of the components included in this assembly, but the water tube alone is also available as a separate Sierra part number if the cover itself does not need replacing.

Do I need to replace the gasket separately when installing this cover? No, the Sierra 18-3219 assembly already includes the gasket along with the water pocket cover, mounting screw, bushing, and water tube, so no separate gasket purchase is needed for this repair.

What are the dimensions of the 18-3219 water pocket cover assembly? The assembly measures 0.39 x 0.39 x 0.39 inches.

How much does the Sierra 18-3219 water pocket cover assembly weigh? It weighs 0.29 pounds.

What finish does the Sierra 18-3219 water pocket cover have? The cover has a machined finish.

Is the Sierra 18-3219 a genuine Sierra part or aftermarket equivalent? The 18-3219 is a Sierra-branded replacement part manufactured to meet or exceed the specifications of the original MerCruiser 42724A3 water pocket cover it replaces.

Why replace the bushing and screw along with the cover instead of just the cover itself? Sierra sells 18-3219 as a full assembly with the mounting screw, gasket, and water pump housing bushing included because these are the components most likely to need renewal at the same time as the water pocket cover, reducing the chance of a repeat leak at the drive shaft housing.

What system does the water pocket cover on part 18-3219 serve? The water pocket cover on the 18-3219 assembly is part of the raw water flow path through the MerCruiser Alpha One/Gen II drive shaft housing, directing cooling water toward the water pump.