Mercury MerCruiser Quicksilver Heat Exchanger to Crossover Seal 865066 | 8.1L 496 MAG
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Mercury / Quicksilver OEM Heat Exchanger to Coolant Crossover Seal — Part No. 865066 / 26-865066
| Part Number | 865066 / 26-865066 |
|---|---|
| Brand | Mercury Marine / Quicksilver |
| Official Description | Seal, Heat Exchanger to Crossover |
| System | Closed / Fresh-Water Cooling |
| Diagram Position | Item 14 — closed-cooling system diagram |
| Quantity Required | 2 per applicable assembly |
| Used With | Heat exchanger 863903A01 and coolant crossover 863771T |
| Supersedes | 8619481 / 26-8619481 |
| Sold As | Individually — order 2 per service |
| UPC / GTIN | 745061561403 |
About This Part
The 865066 / 26-865066 is a genuine Mercury Marine / Quicksilver OEM seal that seals the connection between the heat exchanger (863903A01) and the engine coolant crossover (863771T) on selected MerCruiser 8.1L / 496 CID V8 closed-cooling systems. When the heat exchanger is removed for service or cleaning, both seals must be replaced — a previously compressed seal will not reliably reseal the joint.
Important: This is the heat-exchanger-to-crossover seal — it is not the heat-exchanger end-cover gasket. Confirm you are ordering the correct sealing component by checking your specific closed-cooling diagram before ordering.
Compatible Applications
Confirm the engine model, serial number, and closed-cooling diagram before ordering. Documented applications include selected:
- MerCruiser 496 MAG
- MerCruiser 496 MAG H.O.
- MerCruiser 8.1L / 496 CID V8
- MerCruiser 8.1L Bravo
- MerCruiser 8.1L H.O. Bravo
- MerCruiser 8.1L Inboard
865066 appears across multiple 8.1L / 496 CID closed-cooling configurations including later Quicksilver-manufactured 8.1L engines and earlier 496 MAG / MAG H.O. packages. Fitment is tied to the closed-cooling assembly, not displacement alone — verify against your engine serial number and cooling-system diagram.
When to Replace
- Any time the heat exchanger is removed — both seals are single-use and must be replaced
- Coolant seepage visible at the heat-exchanger-to-crossover joint
- Existing seal is flattened, hardened, cracked, or distorted on inspection
- Coolant crossover (863771T) is being serviced or replaced
- Closed-cooling system is being overhauled or resealed
Installation Notes
- Replace both 865066 seals whenever the heat exchanger is removed — never reuse a previously compressed seal
- Clean the sealing surfaces on both the heat exchanger and the coolant crossover before fitting new seals
- Inspect both mating areas for corrosion, pitting, or distortion
- Inspect heat-exchanger mounting pads 863392 and related hardware while the unit is off
- Check coolant hoses and clamps while access is available — replace any showing cracking or age
- If servicing the cooling system at the same time, also inspect thermostat seal 865078
- Refill and bleed the closed-cooling system per the applicable MerCruiser service procedure
- Pressure-test the cooling system after reassembly before returning to service
Technical FAQ
How many 865066 seals do I need?
Typically two per heat exchanger assembly on applicable 8.1L / 496 engines. Confirm the quantity in your specific closed-cooling diagram before ordering.
What replaced 8619481?
865066 / 26-865066 is the current Mercury part number and supersedes the earlier 8619481 / 26-8619481 reference. If your parts diagram or old invoice shows 8619481, order 865066.
Is this the same as the heat-exchanger end-cover gasket?
No. The 865066 seals the joint between the heat exchanger and coolant crossover. The end-cover gasket is a separate component that seals the removable end cap on the heat exchanger tank. Confirm your diagram position before ordering.
Related Components
The following parts are referenced in the same closed-cooling system diagram on applicable 8.1L / 496 engines:
- 863903A01 — heat exchanger assembly (used with this seal)
- 863771T — coolant crossover (mates with this seal)
- 865078 — thermostat seal (same closed-cooling assembly)
- 863392 — heat exchanger mounting pads
Parts Desk Insights
The most common mistake with this seal is ordering only one when the application requires two. On most 8.1L / 496 systems, there are two seal locations where the heat exchanger meets the coolant crossover — one at each end of the exchanger. If one seal is leaking, both are the same age, so replace the pair at the same time. The other thing we see frequently is customers confusing this seal with the heat-exchanger end-cover gasket — they are different parts at different locations. Check your closed-cooling diagram and identify Item 14 to confirm you have the right component before ordering.
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
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Important:
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