Transom Seal Repair Kit – Fits MerCruiser Alpha One Gen II (Replaces 803099T1, 30-803099T1, 18-8218)
Transom Seal Repair Kit – Fits MerCruiser Alpha One Gen II (Replaces 803099T1, 30-803099T1, 18-8218) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
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Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Transom Seal Repair Kit – Fits MerCruiser Alpha One Gen II (Replace...
Product Details
This comprehensive Transom Seal Repair Kit is designed for MerCruiser Alpha One Gen II stern drives manufactured from 1991 to the present (serial numbers 0D469859 and up). The service kit includes all necessary bellows, gaskets, a gimbal bearing, and hardware required to completely seal the transom opening, protecting the stern drive system from water intrusion and ensuring its operational integrity.
It serves as a direct interchange for Mercury/MerCruiser 803099T1, Sierra 18-8218, Mallory 9-72900, and other equivalent part numbers. Refer to the matrix below for a full list of cross-references.
- Fits Alpha One Gen II models from 1991–Current.
- Includes U-joint, exhaust, and shift cable bellows.
- Comes complete with gimbal bearing, gaskets, and installation hardware.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury / MerCruiser | 803099T1 |
| Mercury / MerCruiser | 30-803099T1 |
| Sierra | 18-8218 |
| Sierra | 18-8206-1 (with greasable bearing) |
| Mallory | 9-72900 |
| Mallory | 9-72906 |
| Red Rhino | MR-907 |
| GLM | 21960 |
| SEI | 9A-116 |
| OBR | 9A-116 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| MerCruiser | Alpha One Gen II | N/A | 4-Stroke | 1991–1995 | 30-803099T1 | Serial Range: 0D469859 through 0F679999 |
| MerCruiser | Alpha One Gen II | N/A | 4-Stroke | 1996–1997 | 30-803099T1 | Serial Range: 0F680000 through 0K999999 |
| MerCruiser | Alpha One Gen II | N/A | 4-Stroke | 1998–Current | 30-803099T1 | Serial Range: 0L100040 and up |
Technical Specs
- Product Type
- Transom Seal Repair Kit
- Installation
- Stern Drive Transom Assembly
- Connection
- Transom to Stern Drive
- Housing Material
- Rubber/Elastomer (Bellows)
- Housing Color
- Black
- Weight
- 2.3 kg (5.0 lb)
- Fitting Type
- Gimbal Housing and Bell Housing
- Operating Temperature
- -18°C to 49°C (0°F to 120°F)
- Warranty
- Varies by Brand (1 year typical)
- Other Technical Attributes
- Serial Numbers 0D469859 and up
What’s in the Box
- 1 x U-Joint Bellow w/ Sleeve & Clamp (Replaces 816431A1)
- 1 x Exhaust Bellow w/ Clamp (Replaces 18654A1/8M0124227)
- 1 x Shift Cable Bellow w/ Clamp (Replaces 74639A2)
- 1 x Gasket Kit (Replaces 94996Q2)
- 1 x Gimbal Bearing (Replaces 879194A02)
- 4 x Grounding Clips
- 1 x Water Hose 3/4" (Replaces 8M0215001)
- 5 x Zip Ties
- 1 x Bell Housing Gasket Set
Installation Notes
- Disconnect the boat's battery and follow all standard marine safety precautions.
- Tilt the stern drive to the trailer position and remove the drive unit from the bell housing according to your service manual.
- Remove the bell housing to access the bellows, gimbal bearing, and water hose.
- Carefully cut and remove the old U-joint, exhaust, and shift cable bellows. Inspect and remove the old gimbal bearing.
- Thoroughly clean all mating surfaces on the transom and bell housing, removing any old sealant or gasket material.
- Install the new gimbal bearing using an appropriate driver tool. Ensure it is properly seated.
- Install the new shift cable bellow, exhaust bellow, and U-joint bellow using the provided clamps. Ensure clamps are tight and properly oriented.
- Connect the new water inlet hose.
- Reinstall the bell housing with the new gasket set, ensuring correct torque specifications.
- Reattach the stern drive unit and check for smooth movement and proper shift cable operation.
- After launching the boat, check for any leaks around the transom assembly before operating at speed.
Troubleshooting & FAQ
How do I confirm this kit fits my Alpha One Gen II drive?
This kit is specifically for MerCruiser Alpha One Gen II drives from 1991 to current. The most reliable way to confirm fitment is by checking your drive's serial number. It fits serial numbers 0D469859 and up. Please verify your serial number before ordering.
What are the signs that my transom seal kit needs replacing?
Common signs of failure include visible cracks or dry rot on the bellows, water inside the bellows (check by squeezing), or water leaking into the bilge area from the transom assembly. A noisy or rough-feeling gimbal bearing when turning the drive is another key indicator.
What is the difference between the Sierra 18-8218 and 18-8206-1 kits?
Both kits are compatible, but the primary difference is the type of gimbal bearing included. The 18-8218 kit typically contains a permanently sealed (non-greasable) bearing, while the 18-8206-1 kit includes a greasable bearing, which requires periodic maintenance.
How often should the bellows be inspected or replaced?
Bellows should be inspected at least annually for cracks, stiffness, or signs of wear. A full replacement is recommended every 3-5 years, depending on usage (saltwater vs. freshwater) and storage conditions, to prevent catastrophic failure.
Is the gimbal bearing in this kit sealed or greasable?
Our standard aftermarket kit, which cross-references Sierra 18-8218, includes a high-quality, permanently sealed gimbal bearing that does not require greasing. This simplifies maintenance but check the specific kit version you order if a greasable type is preferred.
Can I reuse my old clamps or gaskets?
It is strongly recommended to use all new clamps, gaskets, and seals provided in the kit. Reusing old components can lead to improper sealing and water leaks, which can cause severe damage to your transom and drive components.
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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