Shift Shaft Bushing Kit – Fits MerCruiser Alpha One Gen II Stern Drives (Replaces 23-815921A21)
Shift Shaft Bushing Kit – Fits MerCruiser Alpha One Gen II Stern Drives (Replaces 23-815921A21) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
Worry-free returns
You may return most new, unopened items within 7 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect or defective item, etc.).
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Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Shift Shaft Bushing Kit – Fits MerCruiser Alpha One Gen II Stern Dr...
Product Details
This Shift Shaft Bushing Kit is engineered for Mercury MerCruiser Alpha One Generation II stern drives manufactured from 1991 to the present. It serves as a crucial component for the lower unit's shifting mechanism, ensuring smooth gear engagement. The kit is designed to fit a wide range of engine configurations within the Alpha One Gen II series.
It provides a direct interchange for several OEM and aftermarket part numbers, including Sierra 18-2353, Quicksilver 815921A21, and Mercury 23-815921A21/23-815921A20.
- Installs in the lower unit of Alpha One Gen II drives.
- Slides directly over the lower shift shaft for a precise fit.
- Includes the speedometer pickup tube adapter for full functionality.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Sierra | 18-2353 |
| Quicksilver | 815921A21 |
| Mercury | 23-815921A21 |
| Mercury | 23-815921A20 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This kit is specifically for MerCruiser Alpha One Gen II drives. Please verify your drive's serial number falls within the specified range for compatibility.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| MerCruiser | Alpha One Gen II | Various | 4-Stroke | 1991–Present | 23-815921A21 | For drives with serial numbers 0D469859 through 0K999999 |
Technical Specs
- Product Type
- Shift Shaft Bushing Kit
- Installation
- Lower unit of Alpha One Gen II drives
- Connection
- Slides over the lower shift shaft
- Housing Material
- Aluminum
- Housing Color
- White
- Dimensions
- 6.0" × 7.5" × 2.06"
- Weight
- 0.06 kg (0.13 lb)
- Warranty
- 1 year (Mercury/Quicksilver OEM)
- Other Technical Attributes
- Includes speedometer pickup tube adapter
What’s in the Box
- 1 x Shift Shaft Bushing Kit (Replaces 23-815921A21)
- Housing Pieces
- Seals
- Zip Tie
Installation Notes
- Disconnect the negative battery cable before starting work.
- Drain the gearcase lube from the lower unit completely.
- Unbolt and carefully remove the lower unit from the upper drive housing.
- Locate the old shift shaft bushing assembly and remove it from the drive.
- Thoroughly clean the mounting surface, ensuring it is free of old sealant and debris.
- Install the new bushing assembly, making sure the seals are properly seated to prevent leaks.
- Re-attach the lower unit, torquing bolts to manufacturer specifications.
- Refill the gearcase with the correct type and amount of marine gear lube.
- Reconnect the battery and test forward, neutral, and reverse gear engagement before water testing.
Troubleshooting & FAQ
How do I know if this shift shaft bushing is failing?
Common signs of a worn or failed shift shaft bushing include difficulty shifting into forward or reverse, grinding noises during gear changes, or a loose or wobbly feeling in the shift linkage. A visual inspection may also reveal cracks, corrosion, or deteriorated seals on the old part.
Will this fit my specific MerCruiser engine?
This kit is designed for MerCruiser Alpha One Gen II stern drives (1991–Present). The most reliable way to confirm fitment is to verify your drive's serial number is between 0D469859 and 0K999999 and to visually compare your existing part to the images provided.
What is the function of the included speedometer pickup adapter?
The speedometer pickup tube adapter is a small but essential part that connects to the pitot tube in the lower unit. It allows water pressure to be measured as the boat moves, which is then translated into a speed reading on your boat's speedometer.
How often should this part be replaced?
There is no fixed replacement interval for the shift shaft bushing. It should be inspected for wear, damage, or leaks whenever the lower unit is serviced and replaced as needed. It is common to replace it proactively when performing other major drive maintenance.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.
Shipping speed & cost
We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns
Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused & in original condition). See our policy page or reach out for instructions.
Safety & Important Info
- Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended for safety and 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 QC.
Payment & Security
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