Thermostat Kit – Fits Volvo Penta MD19, AQD19, MD21, AQD21 Series (Replaces 875785)
Thermostat Kit – Fits Volvo Penta MD19, AQD19, MD21, AQD21 Series (Replaces 875785) 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 Volvo Penta MD19, AQD19, MD21, AQD21 Series (...
Product Details
This Thermostat Kit – Fits Volvo Penta MD19, AQD19, MD21, AQD21 Series (Replaces 875785) is designed for a range of Volvo Penta diesel marine engines. It is compatible with MD19, AQD19, MD21A, MD21B, AQD21A, AQD21B models from the 1970s–1980s, as well as MD27, MD29, MD32A and their AQD variants.
This kit serves as a direct replacement for Volvo Penta part numbers 875785/833826, Sierra 18-3659, and Mallory 9-43012.
- Engineered for direct-fit installation.
- Rated for a precise 71°C (160°F) operating temperature.
- Includes essential components for a complete replacement.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket brands.
| Brand | Part Number |
|---|---|
| Volvo Penta | 875785 |
| Volvo Penta | 833826 |
| Sierra | 18-3659 |
| Mallory | 9-43012 |
Engine & Model Compatibility
Confirm your engine model and year in the table below to ensure accurate fitment.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. |
|---|---|---|---|---|---|
| Volvo Penta | AQD21A | 75 | 4-Stroke | 1969–1978 | 875785 |
| Volvo Penta | AQD21B | 75 | 4-Stroke | 1979–1981 | 875785 |
| Volvo Penta | MD21A | 75 | 4-Stroke | 1969–1978 | 875785 |
| Volvo Penta | MD21B | 75 | 4-Stroke | 1979–1981 | 875785 |
| Volvo Penta | AQD32A | 106 | 4-Stroke | 1969–1973 | 875785 |
Technical Specs
- Product Type
- Thermostat Kit
- Installation
- Direct Replacement
- Temperature Rating
- 71°C (160°F)
- Dimensions
- 7.5" x 6.0" x 2.25"
- Warranty
- Limited Lifetime (Sierra aftermarket version)
- Country of Manufacture
- Israel (Sierra aftermarket version)
What’s in the Box
- 1 x Thermostat Kit (Replaces 875785/833826)
Installation Notes
- Ensure the engine is completely cool before starting work.
- Drain the engine coolant to a level below the thermostat housing.
- Locate and remove the thermostat housing cover bolts.
- Carefully remove the old thermostat and gasket. Note the orientation of the old thermostat.
- Clean both the housing cover and the engine block mating surfaces thoroughly.
- Install the new thermostat in the correct orientation, followed by the new gasket/seal.
- Reinstall the housing cover and tighten the bolts to the manufacturer's specified torque.
- Refill the cooling system with the appropriate coolant.
- Start the engine and check for leaks as it warms up to operating temperature.
Troubleshooting & FAQ
Which specific Volvo Penta engines does this thermostat fit?
This kit is compatible with Volvo Penta models MD19, AQD19, MD21A/B, AQD21A/B, MD27, AQD27, MD29, AQD29/A, and MD32A/AQD32A. Always check the compatibility table to confirm your specific model.
What are the signs of a faulty thermostat?
Common symptoms include engine overheating, the engine not reaching normal operating temperature, erratic temperature gauge readings, and visible coolant leaks around the thermostat housing.
How often should a marine engine thermostat be replaced?
Thermostats are a critical maintenance item. While there's no fixed interval, it's good practice to inspect them annually and consider replacement every 3-5 years or as recommended by your engine service manual, especially in saltwater environments.
Does this kit include the necessary gaskets or seals?
Yes, this is a kit that includes the thermostat and the required seal or gasket for a standard installation. What you see in the product photos is what you will receive.
Is there a difference in performance between the 71°C and other temperature thermostats?
Yes. The temperature rating is specified by the engine manufacturer to ensure optimal performance, fuel efficiency, and longevity. Using a thermostat with a different temperature rating can negatively impact your engine.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). It is built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, it is batch-inspected for consistency and quality.
Shipping speed & cost
We ship worldwide quickly—typical transit is 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 and specifications against your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement component, not an OEM part.
- 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 quality control.
Payment & Security
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