Mercury MerCruiser OEM Heat Exchanger End Cover 66221 | Closed Cooling | 3.0L 5.0L 5.7L 7.4L

Mercury MerCruiser OEM Heat Exchanger End Cover — Part 66221

• Part Number: 66221
• Brand: Mercury Marine / MerCruiser (Genuine OEM)
• Official Product Type: Heat Exchanger & Closed Cooling Components — Heat Exchanger Tank End Cover
• System: Cooling — closed-cooling heat exchanger / coolant reservoir, end cover sealing
• Diagram Position: End cover at the heat exchanger tank, used with end-cover gasket 65128 and fiber washer 75451
• Used With: End-cover gasket 65128, end-cover bolt 66223, fiber washer 75451
• GTIN / UPC: 745061168282
• Weight: approx. 0.15 kg

Quantity note: Some MerCruiser heat-exchanger assemblies use two 66221 end covers — one at each end of the tank. Check the closed-cooling diagram for your specific engine serial number to confirm the required quantity before ordering.

What It Does

The 66221 is the genuine OEM heat exchanger tank end cover used in select MerCruiser closed-cooling systems. It mounts at the end of the heat exchanger or coolant reservoir assembly and works with the corresponding end-cover gasket, retaining bolt, and fiber washer to seal the heat-exchanger tank. Mercury diagrams identify 66221 specifically as the heat exchanger end cover, not a general engine or timing cover. If coolant is leaking at the end of the heat exchanger, the cover, gasket, and heat-exchanger sealing surface should all be inspected — a new cover alone will not correct a leak caused by a damaged tank sealing surface or a heavily compressed gasket.

Compatible Applications

Confirm fitment and required quantity by engine serial number and the specific MerCruiser closed-cooling diagram before ordering. Mercury used several different heat-exchanger and closed-cooling configurations within the same engine families.

• MerCruiser 3.0L / 181 CID — selected closed-cooling configurations
• MerCruiser 5.0L / 305 CID V8 — selected closed-cooling configurations
• MerCruiser 5.7L / 350 CID V8 — selected closed-cooling configurations
• MerCruiser 7.4L / 454 CID V8 — selected closed-cooling configurations
• Selected 230 HP 5.0L configurations
• Selected 260 HP 5.7L configurations
• Several complete MerCruiser closed-cooling heat-exchanger and reservoir assemblies that include 66221

Not sure it fits or how many you need? Send us your MerCruiser engine serial number before ordering.

Related Closed-Cooling Assembly Components

The 66221 end cover is commonly installed with the following components. Inspect all of them during heat-exchanger service — these are sold separately and are not included with the cover:

• 65128 — Heat exchanger end-cover gasket (replace whenever the cover is disturbed)
• 66223 — End-cover retaining bolt
• 75451 — Fiber washer (used with retaining hardware)
• 328022 — 1/8-27 brass pipe plug (heat-exchanger tank plug on applicable assemblies)
• 78094A3 — Closed-cooling reservoir assembly (uses two 66221 end covers on applicable systems)
• 8M0157284 — Heat exchanger assembly (related closed-cooling configuration)

Assembly example: MerCruiser reservoir assembly 78094A3 uses 2 × 66221 end covers, 2 × 65128 gaskets, 2 × 75451 fiber washers, and 1 × 328022 brass pipe plug. Confirm the correct quantity from the diagram for your specific assembly.

Part Numbers

• 66221 — Mercury Marine / MerCruiser OEM part number

No separate superseding Mercury part number is confirmed in current parts references. Order 66221.

When to Replace

Replace the heat exchanger end cover when:

• Corrosion or pitting has damaged the cover or its sealing surface
• The cover is cracked, warped, or distorted
• Coolant is leaking around the end of the heat exchanger
• The cover has been damaged during heat-exchanger service or removal
• The retaining-bolt boss area is cracked or worn
• The cover can no longer compress the gasket evenly across the sealing face

Service Interval

Inspect the end cover and gasket at each closed-cooling system service and whenever the heat exchanger is opened. The end-cover gasket (65128) should be replaced any time the cover is removed — a compressed or disturbed gasket will not reseal reliably. If coolant shows signs of contamination or discoloration, pull the heat exchanger and inspect the end covers, gaskets, and tank sealing surfaces before the next season. For engines operated in saltwater, galvanic corrosion around the end-cover seat and tank plug should be checked at each annual cooling-system service.

Installation Notes

• Confirm 66221 is identified as the correct end cover at the applicable location in the MerCruiser closed-cooling diagram for your engine serial number, and confirm the required quantity (one or two) before ordering
• Replace the end-cover gasket (65128) whenever the cover is removed — do not reuse a disturbed gasket
• Inspect the fiber washer (75451) and replace if compressed or damaged
• Clean all corrosion from the heat-exchanger tank sealing surface before fitting the replacement cover — a pitted or corroded sealing surface will prevent the new cover and gasket from sealing correctly
• Inspect the heat-exchanger tank for pitting around the cover seat; a heavily corroded tank sealing surface cannot be corrected by a new cover and gasket alone
• Follow the applicable MerCruiser service procedure for torque on the end-cover retaining hardware
• Check for coolant leaks carefully after reassembly before returning the engine to service

Technical FAQ

Q: How many 66221 end covers does my heat exchanger need?
It depends on the specific heat-exchanger or reservoir assembly for your engine serial number. Some assemblies use one end cover; others (such as reservoir assembly 78094A3) use two. Check the MerCruiser closed-cooling diagram for your engine before ordering to confirm the correct quantity.

Q: Can I reuse the gasket when I replace the end cover?
No. The end-cover gasket (65128) should always be replaced when the end cover is removed. A gasket that has been compressed in service will not reseal reliably after being disturbed.

Q: My heat exchanger is leaking at the end cover — is a new cover all I need?
Not necessarily. Leakage at the end cover can be caused by a failed gasket, a corroded or pitted tank sealing surface, a cracked cover, or a combination of these. Inspect the tank sealing surface and the existing gasket when the cover is removed. A new cover and gasket will not seal correctly against a badly corroded or pitted tank surface.

Q: Does 66221 fit both ends of the heat exchanger if I have two covers?
Yes, on assemblies that use two end covers, 66221 is the correct part for both positions. Confirm the quantity from the MerCruiser closed-cooling diagram for your engine serial number and order accordingly.

Parts Desk Insights

The most common issue with 66221 is customers ordering one end cover when their assembly uses two. If the closed-cooling diagram shows covers at both ends of the heat-exchanger tank, order both at once — the gasket (65128) should be replaced at both locations at the same time. The other thing that comes up regularly is coolant leaks that come back after a cover and gasket replacement: in most of these cases the heat-exchanger tank sealing surface is pitted from corrosion and the new gasket cannot seal against it. Inspect and clean the tank sealing surface thoroughly before fitting the new cover, and advise the customer to have the tank evaluated if corrosion is significant.