Water Pump Impeller Kit – Fits Mercury R/MR/Alpha One (Replaces 42724A3)
Water Pump Impeller Kit – Fits Mercury R/MR/Alpha One (Replaces 42724A3) is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
Delivery and Shipping
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Water Pump Impeller Kit – Fits Mercury R/MR/Alpha One (Replaces 427...
Water Pump Impeller Kit – Fits Mercury R/MR/Alpha One (Replaces 42724A3) is backordered and will ship as soon as it is back in stock.
Product Details
This Water Pump Impeller Kit is designed for Mercury Marine R, MR, and Alpha One Generation I sterndrives manufactured between 1984 and 1990 (serial numbers 6854393 to OD469858). It provides the essential components for servicing the lower unit's water pump.
It serves as a direct interchange for several original and aftermarket part numbers, including Mercury 42724A3, Mercury 46-37433A1, Sierra 18-3219, and GLM 12200.
- Engineered as a direct replacement for a straightforward installation.
- Compatible with Generation 1 (Alpha I) and MR model Mercruiser lower unit outdrives.
- Includes the impeller, pocket cover, and guide tube for a complete service.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket brands.
| Brand | Part Number |
|---|---|
| Mercury | 42724A3 |
| Mercury | 46-37433A1 |
| Sierra | 18-3219 |
| GLM | 12200 |
Engine & Model Compatibility
Confirm your engine model and year in the table below to ensure accurate fitment.
For Mercury Marine engines, this kit is specifically designed for the R, MR, and Alpha One (Gen I) sterndrive models within the specified serial number ranges.
| Brand | Model | Years | OEM Part No. | Notes |
|---|---|---|---|---|
| Mercury | R | 1967–1990 | 42724A3 | For serial numbers 2495186–OD469858 |
| Mercury | MR | 1967–1990 | 42724A3 | For serial numbers 2495186–OD469858 |
| Mercury | Alpha One Gen I | 1984–1990 | 42724A3 | For serial numbers 6854393–OD469858 |
Technical Specs
- Product Type
- Water Pump Impeller Kit
- Installation
- Direct replacement
- Compatibility
- Compatible with Generation 1 (Alpha I) and MR model Mercruiser lower unit outdrives
What’s in the Box
- 1 x Water Pump Impeller Kit (Replaces 42724A3) including:
- 1 x Pocket Cover (42724A3)
- 1 x Guide Tube (23-29804T)
Installation Notes
- Disconnect the negative battery cable for safety.
- Shift the sterndrive into the forward gear position.
- Remove the lower unit (gearcase) from the upper housing by undoing the main bolts.
- Unbolt and carefully remove the old water pump housing assembly from the lower unit.
- Thoroughly clean the mating surfaces of any old gasket material or debris.
- Assemble the new impeller, key, and plate from the kit into the new housing.
- Apply a light coat of marine grease to the inside of the pump housing and the impeller vanes.
- Install the new pump assembly onto the driveshaft. Gently turn the shaft clockwise to help seat the impeller vanes in the correct direction.
- Securely bolt the new pump assembly in place, tightening to the manufacturer's torque specifications.
- Reinstall the lower unit, ensuring the water tube and shift shaft are properly aligned.
- Reconnect the battery, supply cooling water to the drive, and briefly run the engine to check for proper water flow.
Troubleshooting & FAQ
How do I confirm if this kit fits my sterndrive?
This kit is specifically for Mercury R, MR, and Alpha One Generation I drives from 1984–1990, within serial numbers 6854393 to OD469858. Always verify your sterndrive model and serial number range against the compatibility table before ordering.
What are the signs of a bad water pump impeller?
The most common symptoms are engine overheating, especially at idle, a weak or non-existent water stream from the tell-tale outlet, and a dry, cracked, or brittle appearance of the old impeller upon inspection.
How often should I replace my sterndrive's impeller?
For preventative maintenance, it is recommended to replace the impeller every 1 to 2 years or every 100-200 hours of operation. Frequency should be increased if you operate in saltwater or water with high levels of sand or silt.
Does this kit include the full pump housing?
This kit includes the key service components: the impeller, a pocket cover, and a guide tube. Please refer to the "What’s in the Box" section and product images to ensure it matches your specific repair needs.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). It is built to OEM-level specifications with comparable materials and manufactured in an ISO 9002 facility, where it is batch-inspected for consistency.
Shipping speed & cost?
We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free, and expedited options may appear at checkout where available.
Warranty & returns?
Our parts are covered by a 1-year warranty against manufacturing defects under normal use. If an issue arises, contact our support team with your order details. Returns are accepted within our standard window for unused items in original condition. See our policy page or reach out for instructions.
Safety & Important Info
- Verify images and specifications against your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement part, not an OEM component.
- Professional installation is recommended for safety and optimal performance.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
- Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
- Manufacturer-Direct Value — Fair pricing without compromising on materials or quality control.
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