Cylinder Head Gasket – Fits Mercury/Mariner 135-150HP Outboards (Replaces 27-18785T)

$44.99 $71.00

Shipping calculated at checkout.

Cylinder Head Gasket – Fits Mercury/Mariner 135-150HP Outboards (Replaces 27-18785T) is backordered and will ship as soon as it is back in stock.

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 Cylinder Head Gasket – Fits Mercury/Mariner 135-150HP Outboards (Replaces 27-18785T) offers a precision-engineered seal for Mercury and Mariner V6 2.0L Loop-Charged outboards manufactured between 1988 and 2000. Made from high-quality materials, it restores engine integrity and performance.

This part serves as a direct interchange for Mercury numbers 27-18785T, 27-18785-1, and 27-187853, and is cross-compatible with aftermarket equivalents from GLM (35850), Sierra (18-3861), and Mallory (9-63851).

Manufactured from multi-layer steel for superior durability and heat resistance.
Engineered for direct replacement, ensuring a precise fit and seal.
Designed for use with engines featuring a 3.125" (79.38 mm) piston diameter.

Part Number Matrix

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

Brand	Part Number
Mercury	27-18785T
Mercury	27-18785-1
Mercury	27-187853
GLM	35850
Sierra	18-3861
Mallory	9-63851
Asaki	Aftermarket

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercury, Mariner, and Force models, check your engine's serial number against the ranges provided to ensure an exact match.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury/Mariner	V-135	135	2-Stroke	1991–1998	27-18785T	Serial Range: 0C239553-0T178499
Mercury/Mariner	V-150	150	2-Stroke	1988–1991	27-18785T	Serial Range: 0C239553-0G960499
Mercury/Mariner	XR-4	135-150	2-Stroke	1992–2000	27-18785T	V6 2.0L Loop-Charged
Mercury/Mariner	XR-6	150-200	2-Stroke	1992–2000	27-18785T	V6 2.0L Loop-Charged
Mercury/Mariner	MAGNUM II	135-150	2-Stroke	1991–1998	27-18785T	V6 2.0L
Mercury/Mariner	MAGNUM III	135-150	2-Stroke	1992–2000	27-18785T	V6 2.0L
Mercury/Force	105 Jet (V6)	105	2-Stroke	1996 & Up	27-18785T	Serial Range: 0D082000 and Up
Mercury/Force	140 Jet	140	2-Stroke	1992–2000	27-18785T	V6 2.0L Loop-Charged

Technical Specs

Product Type: Cylinder Head Gasket
Installation: Direct replacement
Fuel Type: Gasoline
Housing Material: Multi-layer steel
Dimensions: 42.0 × 14.0 × 0.5 in (approx. 1067 × 356 × 13 mm)
Weight: 0.14 kg (0.31 lb)
Piston Diameter: For use with 3.125" (79.38 mm) pistons

What’s in the Box

1 x Head Gasket (Replaces 27-18785T)

Installation Notes

Ensure the engine is cool and disconnect the battery.
Drain the engine coolant to below the cylinder head level.
Disconnect all electrical wiring, hoses, and linkages attached to the cylinder head.
Remove the intake and exhaust manifolds from the head.
Following your service manual, loosen and remove the head bolts in the specified reverse sequence.
Carefully lift the cylinder head from the engine block. Avoid prying on mating surfaces.
Remove the old gasket. Thoroughly clean all residue from both the block and head surfaces with an appropriate solvent.
Position the new gasket over the alignment dowels on the engine block. Do not use sealant unless explicitly required by the gasket manufacturer.
Lower the cylinder head back into place without disturbing the gasket.
Install new or inspected head bolts. Tighten them in the correct sequence and to the precise torque values listed in your service manual. This step is critical.
Reinstall all previously removed components.
Refill the cooling system, reconnect the battery, and start the engine. Check for any coolant, fuel, or exhaust leaks.

Troubleshooting & FAQ

How do I confirm this gasket fits my engine?

The best method is to match your engine's model, year, and serial number to our compatibility chart. The serial number ranges are key for ensuring a correct fit, especially for Mercury and Mariner models. If in doubt, compare our product images and specifications to your old part.

What are the signs of a blown head gasket?

Common symptoms include engine overheating, white smoke from the exhaust (burning coolant), milky or foamy oil, unexplained coolant loss, and poor engine performance or misfires. A compression test can help confirm the diagnosis.

How often should a head gasket be replaced?

Head gaskets are not a routine maintenance item. They are replaced only when they fail. Their lifespan depends on engine condition, operating habits, and preventing overheating.

Can I reuse my old head bolts?

It is strongly recommended to use new head bolts. Many are "torque-to-yield" (TTY) bolts designed to stretch permanently upon tightening and cannot provide the correct clamping force if reused, leading to premature gasket failure.

Is any sealant required when installing this gasket?

No. This multi-layer steel (MLS) gasket is designed to be installed dry. Applying sealant can interfere with its ability to create a proper seal and may cause it to fail. Always ensure mating surfaces are perfectly clean and dry.

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.