Thermostat Kit – Fits Mercury/Mariner 70-125 HP 2-Stroke Outboards (Replaces 803061T1)
Thermostat Kit – Fits Mercury/Mariner 70-125 HP 2-Stroke Outboards (Replaces 803061T1) 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.).
You should expect to receive your refund within four weeks of giving your package to the return shipper, however, in many cases you will receive a refund more quickly. This time period includes the transit time for us to receive your return from the shipper (5 to 10 business days), the time it takes us to process your return once we receive it (3 to 5 business days), and the time it takes your bank to process our refund request (5 to 10 business days).
If you need to return an item, simply login to your account, view the order using the "Complete Orders" link under the My Account menu and click the Return Item(s) button. We'll notify you via e-mail of your refund once we've received and processed the returned item.
Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Thermostat Kit – Fits Mercury/Mariner 70-125 HP 2-Stroke Outboards ...
Product Details
This Thermostat Kit – Fits Mercury/Mariner 70-125 HP 2-Stroke Outboards (Replaces 803061T1) is an aftermarket replacement part for Mercury/Mariner 70-125 HP 2-Stroke inline 3 & 4 cylinder outboards (1987–2010). It is designed to regulate engine temperature for optimal cooling system performance.
This kit serves as a direct cross-reference for Mercury 803061T1, Sierra 18-3569, Sierra 18-3549, GLM 13124, and Mallory 9-43155, among others.
- Engineered for direct OEM replacement fitment.
- Maintains engine operating temperature at 122°F / 50°C.
- Includes thermostat and necessary gaskets for a complete installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury | 803061T1 |
| Sierra | 18-3569 |
| Sierra | 18-3549 |
| Sierra | 18-0339 |
| GLM | 13124 |
| Mallory | 9-43155 |
| Mallory | 9-43014 |
| Mallory | 9-60004 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This thermostat kit is verified for the following Mercury 2-stroke outboard models.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Mercury | 70 | 70 | 2-Stroke | 1987–2004+ | 803061T1 | 3 Cylinder |
| Mercury | 75 | 75 | 2-Stroke | 1990–2010 | 803061T1 | 3 Cylinder |
| Mercury | 80 | 80 | 2-Stroke | 1987–1990 | 803061T1 | 3 Cylinder |
| Mercury | 90 | 90 | 2-Stroke | 1987–2007+ | 803061T1 | 3 Cylinder |
| Mercury | 100 | 100 | 2-Stroke | 1987–2004+ | 803061T1 | 4 Cylinder |
| Mercury | 115 | 115 | 2-Stroke | 1987–2004+ | 803061T1 | 4 Cylinder |
| Mercury | 125 | 125 | 2-Stroke | 1987–2004+ | 803061T1 | 4 Cylinder |
Technical Specs
- Product Type
- Thermostat Kit
- Installation
- Direct OEM replacement
- Temperature Rating
- 122°F / 50°C
- Housing Material
- Brass/Copper
- Dimensions
- 11.25 × 6.0 × 1.0 inches
- Weight
- 0.13 lb (0.059 kg)
- Warranty
- 1 Year (aftermarket versions)
- Other Technical Attributes
- For cooling system maintenance
What’s in the Box
- 1 x Thermostat (122°F/50°C)
- 1 x Thermostat Gasket
- 1 x Washer
- 1 x Cover Gasket
Installation Notes
- Ensure the engine is cool and the battery is disconnected.
- Drain the engine coolant to a level below the thermostat housing.
- Locate and remove the bolts securing the thermostat housing cover.
- Carefully remove the old thermostat, gaskets, and washer, noting their orientation.
- Thoroughly clean the mating surfaces of the housing and cover to remove all old gasket material.
- Install the new thermostat, gasket, and washer in the housing, ensuring they are correctly aligned.
- Reattach the housing cover and tighten the bolts to the torque specified in your service manual.
- Refill the cooling system, restart the engine, and carefully check for any leaks.
Troubleshooting & FAQ
How do I know if my thermostat has failed?
Common signs include the engine overheating, especially at idle or low speeds, or taking too long to reach its proper operating temperature. An erratic temperature gauge reading can also indicate a faulty thermostat.
Is this the correct temperature rating for my engine?
This thermostat is rated for 122°F / 50°C, which is the specified temperature for the listed Mercury/Mariner models to ensure efficient operation and prevent overheating.
Can I use this kit in a 4-stroke engine?
No. This kit is designed exclusively for the 2-Stroke inline 3 & 4 cylinder outboards detailed in the compatibility table. Using it in an incompatible engine may lead to poor performance or damage.
What is the recommended replacement interval for a thermostat?
For preventative maintenance, it is advisable to inspect your thermostat annually and replace it every 2-3 years, or whenever you service the water pump. Replace it immediately if you suspect a failure.
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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