Gasket – Fits Mercury Marine/Mercruiser 35-40 HP Outboards (Replaces 27-856729)

$8.99

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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 block cover Gasket – Fits Mercury Marine/Mercruiser 35-40 HP Outboards (Replaces 27-856729) and is engineered for various years of 35-40 HP, 2-Stroke outboard engines. It ensures a reliable seal for the cylinder block crankcase and covers.

It serves as a direct replacement for Mercury Marine 27-856729 and cross-references with other OEM and aftermarket parts including Sierra 18-2805 and GLM 31920.

Engineered for a direct fit to simplify installation.
Matches or exceeds OEM factory original equipment standards.
Precision-cut to ensure a tight, leak-free seal.

Part Number Matrix

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

Brand	Part Number
Mercury Marine	27-856729
Mercury	27-55307
Mercury	27-74708
Mercury	27-78029
Mercury	27-85490
Mercury	27-85490-2
Mercury	27-854901
Sierra	18-2805
GLM	31920

Engine & Model Compatibility

Confirm your engine model/year in the table below.

This gasket is verified to fit the following Mercury Marine/Mercruiser outboard engine models.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury	402, 40 (2 CYL.)	40	2-Stroke		27-856729	CYLINDER BLOCK CRANKCASE AND COVERS
Mercury	35	35	2-Stroke		27-856729	CYLINDER BLOCK CRANKCASE AND COVERS
Mercury		35-40	2-Stroke	1979	27-856729	Verified fit for 1979 outboard

Technical Specs

Product Type: Block Cover Gasket
Installation: Direct fit
Dimensions: Height: 11.63 in, Width: 8.0 in, Length: 1.13 in
Weight: 0.03 lb (0.014 kg)
Other Technical Attributes: OEM factory original equipment

What’s in the Box

1 x Gasket (27-856729)

Installation Notes

Ensure the engine is off and cool, then disconnect the battery.
Drain any necessary fluids according to your service manual.
Remove any components obstructing access to the block cover.
Carefully unbolt and remove the old cover and gasket.
Thoroughly clean all old gasket material and residue from both mating surfaces until they are smooth and clean.
Position the new gasket correctly on the engine block.
Reinstall the cover, tightening bolts to the manufacturer’s specified torque and sequence.
Reassemble all components that were previously removed.
Refill engine fluids and reconnect the battery.
Start the engine and carefully inspect for leaks around the newly installed gasket.

Troubleshooting & FAQ

How can I be sure this gasket fits my specific engine model?

This gasket is designed for Mercury 35-40 HP 2-stroke outboards. To confirm, cross-reference your existing part number with our Part Number Matrix, and check the model compatibility table. Always visually compare the new part to your old one before installation.

What is the most important step when installing this gasket?

The most critical step is surface preparation. Both the engine block and the cover mating surfaces must be perfectly clean, dry, and free of any old gasket material, oil, or debris to ensure a proper seal.

What are the signs of a failing block cover gasket?

Common symptoms include external oil or coolant leaks from the side of the engine block, engine overheating, or a loss of compression, which can lead to poor performance.

How often should this gasket be replaced?

This gasket does not have a set replacement interval. It should be replaced whenever the block cover is removed for service or if signs of a leak or failure are detected.

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.

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