Powerhead Base Gasket – Fits Mercury Mariner Outboard 35 HP (27-628601)

$9.50

Shipping calculated at checkout.

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

Free standard shipping on all orders under 1 KG

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 aftermarket Powerhead Base Gasket is a crucial sealing component for Mercury Mariner 35 HP outboard engines manufactured from 1987 and later, including models in the B110789 and up series.

It provides a direct interchange for Sierra part 18-0988 and Mercury/Quicksilver references 27-628601 and 628601.

Installs directly between the powerhead and the exhaust extension plate.
Made from durable, heat-resistant materials to ensure a reliable seal.
Package includes one gasket for a complete replacement.

Part Number Matrix

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

Brand	Part Number
Sierra	18-0988
Mercury	27-628601
Mercury	628601
Quicksilver	27-628601

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercury and Mariner outboards, verify your model and serial number range to ensure correct fitment.

Brand	Model	Rated HP	Years	OEM Part No.	Notes
Mercury	Mariner	35	1987 and later	27-628601	For models 9466647 & Up
Mercury	Mariner	35	1987 and later	27-628601	For models B110789 & Up

Technical Specs

Product Type: Powerhead Base Gasket
Installation: Direct replacement, placed between powerhead and exhaust extension plate
Fuel Type: Gasoline
Housing Material: Heat-resistant gasket material
Dimensions: 13.0 in x 10.0 in x 1.13 in
Weight: 142g (5 oz)

What’s in the Box

1 x Gasket (27-628601)

Installation Notes

Ensure the engine is off and cool, and disconnect the battery before starting.
Remove the engine cowling and any other components obstructing access to the powerhead.
Systematically unfasten the bolts securing the powerhead to the exhaust extension plate.
Carefully lift the powerhead away and set it aside securely.
Remove the old powerhead base gasket. Thoroughly clean both mating surfaces to remove all old gasket material, sealant, and debris. A clean, smooth surface is critical for a proper seal.
Position the new gasket onto the exhaust extension plate, ensuring it aligns perfectly with all bolt holes and passages.
Carefully lower the powerhead back into place. Re-install the bolts and tighten them in a crisscross pattern to the manufacturer's specified torque values.
Reassemble all previously removed components, reconnect the battery, and start the engine, checking carefully for any exhaust or water leaks around the new gasket.

Troubleshooting & FAQ

How do I confirm this gasket fits my Mercury 35 HP engine?

This gasket fits Mercury/Mariner 35 HP models from 1987 and later. The most reliable way to confirm fitment is to check your engine's serial number. This part is compatible with models in the "9466647 & Up" and "B110789 & Up" series. Please refer to the Engine Compatibility table on this page.

What are the signs of a failing powerhead base gasket?

Common symptoms of a bad gasket include engine overheating, loss of power or compression, water intrusion into the cylinders (milky oil), or visible exhaust/water leaks between the powerhead and the exhaust plate.

Do I need to use sealant or adhesive with this gasket?

This gasket is designed to create a seal on its own. It is generally not recommended to use additional sealants unless explicitly required by your engine's official service manual for your specific model.

When should this gasket be replaced?

The powerhead base gasket should be replaced whenever the powerhead is removed for service or during a major engine rebuild. It should also be replaced immediately if you notice any signs of 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

Payment methods

Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.