SeaSierra

Head Cover Gasket – Fits Mercury/Johnson/Evinrude 20 HP (Replaces 27-78410, 508810)

$5.99

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Delivery and Shipping

Orders are typically processed within 1–2 business days.

U.S. orders usually arrive within 6–15 days, depending on stock location and destination.

International delivery times may vary due to customs processing.

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If you have any questions, feel free to email us at info@sierramarines.com, and we’ll get back to you within 20–36 hours.

Product Details

This high-quality Head Cover Gasket is designed to provide a reliable seal for a range of outboard motors, including specific Mercury, Johnson/Evinrude, and Chrysler 2-stroke models from 1978–1980. It ensures a precise fit to prevent leaks and maintain engine performance.

This gasket serves as a direct replacement for multiple OEM and aftermarket part numbers, including Mercury 27-78410, 52579, 72916, 78410; OMC/Johnson/Evinrude 508810; and Sierra 18-0372.

Designed for direct OEM replacement installation.
Engineered to match the fit and function of original parts.
Includes one head cover gasket per package.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Brand	Part Number
Mercury	27-78410
Mercury	52579
Mercury	72916
Mercury	78410
OMC/Johnson/Evinrude	508810
Sierra	18-0372

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercury engines, this gasket fits select 20 HP and 9.9 HP 2-stroke models from 1978–1980.

For Johnson/Evinrude, it is compatible with various 20 HP 2-stroke models between 1978 and 1980.

For Chrysler, compatibility is noted for certain 9.9 HP 2-stroke models from 1978–1980.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury	200, 20	20	2-Stroke	1978–1980	27-78410	Also listed as 52579, 72916, 78410
Mercury	9.9	9.9	2-Stroke	1980	27-78410	Models 95B0F, 95H0F
Johnson/Evinrude	Various	20	2-Stroke	1978–1980	508810
Chrysler	Various	9.9	2-Stroke	1978–1980		Compatibility noted in Green Bay Propeller listing

Technical Specs

Product Type: Head Cover Gasket
Installation: Direct OEM replacement
Product Dimensions: 11.25" L x 6.0" W x 1.13" H
OEM Part Numbers: Mercury: 27-78410, 52579, 72916, 78410
OMC/Johnson/Evinrude: 508810
Aftermarket Part Number: Sierra 18-0372

What’s in the Box

1 x Head Cover Gasket

Installation Notes

Ensure the engine is off and cool to the touch. Disconnect the spark plug wires and battery for safety.
Remove the engine cowling to access the cylinder head cover.
Carefully and evenly loosen and remove the bolts securing the head cover. Note their positions if they vary in length.
Gently pry off the cover. Avoid using tools that could gouge the metal surfaces.
Remove the old gasket and discard it.
Thoroughly clean all mating surfaces on both the engine block and the head cover. Use a plastic scraper or appropriate solvent to remove all traces of old gasket material.
Position the new gasket correctly. Do not use sealant unless explicitly required by your engine's service manual.
Reinstall the head cover. Hand-tighten the bolts, then tighten them in the sequence and to the torque specifications recommended by the manufacturer.
Reconnect the spark plug wires and battery, then check for leaks after starting the engine.

Troubleshooting & FAQ

How do I know if this gasket fits my specific engine model?

Always verify your engine's model, year, and the OEM part number from your original part against the information in our compatibility tables. If your part number or model is not listed, this gasket may not fit.

What are the signs of a failing head cover gasket?

Common signs include visible oil leaks around the head cover seam, a smell of burning oil, engine overheating, or a noticeable loss of performance and compression.

Does this gasket require sealant during installation?

In most cases, this gasket is designed to be installed dry. Always consult your engine's official service manual for specific instructions, as some models may have different requirements.

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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Head Cover Gasket – Fits Mercury/Johnson/Evinrude 20 HP (Replaces 27-78410, 508810)

Worry-free returns

Delivery and Shipping