Washer – Fits Mercury/Mariner 15-50 HP (Replaces 12-86645, 12-86645-1, 12-8M0027721)

$3.99

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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).

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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 high-quality steel Washer – Fits Mercury/Mariner 15-50 HP is designed for a wide range of 2-stroke and 4-stroke outboard engines. It is a critical component used in water pump and gearcase assemblies to ensure a proper seal and alignment.

It serves as a direct replacement for Mercury/Mariner OEM parts 12-86645, 12-86645-1, and 12-8M0027721. It also cross-references with Sierra 18-2560, GLM 31210, and OMC/Johnson/Evinrude 0509368.

Installs between the impeller and the driveshaft or pump housing.
Essential for routine water pump service and maintenance.
Constructed from durable steel to resist corrosion and wear.

Part Number Matrix

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

Brand	Part Number	Superseded By	Notes
Mercury/Mariner	12-86645	12-86645-1	Original Part
Mercury/Mariner	12-86645-1	12-8M0027721	Superseded Part
Mercury/Mariner	12-8M0027721	12-8M0204722	Current active part number.
Sierra	18-2560		Aftermarket Interchange
GLM	31210		Aftermarket Interchange
OMC/Johnson/Evinrude	0509368		Aftermarket Interchange

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercury/Mariner engines, this washer is frequently used in the gearcase and driveshaft assemblies of 2-stroke and 4-stroke models ranging from 15 HP to 50 HP.

Brand	Model	Rated HP	Stroke	OEM Part No.	Notes
Mercury/Mariner	15/18XD, 20, 25 HP	15-25	2-Stroke	12-86645	Used in gearcase assembly
Mercury/Mariner	25-50 HP	25-50	2-Stroke	12-86645	Used in gearcase assembly (1997–2006)
Mercury/Mariner	30, 40 HP (Carb 3 cyl)	30-40	4-Stroke	12-86645	Used in gearcase assembly
Mercury/Mariner	30, 40 HP (EFI)	30-40	4-Stroke	12-86645	Used in gearcase assembly
Mercury/Mariner	45 Bodensee	45		12-86645	Used in gearcase assembly
Mercury/Mariner	50 HP	50	4-Stroke	12-86645	Used in gearcase assembly

Technical Specs

Product Type: Impeller Washer
Installation: Between impeller and driveshaft/pump housing
Housing Material: Steel
Housing Color: Silver/Metallic
Product Dimensions: 4.0" H x 5.13" W x 1.13" L
Other Technical Attributes: Flat washer used in water pump assemblies

What’s in the Box

1 x Washer (Replaces 12-86645, 12-86645-1, 12-8M0027721)

Installation Notes

Disconnect the negative battery cable and remove the propeller.
Position a drain pan under the lower unit and remove the gearcase oil drain/fill screws to empty the oil.
Remove the bolts securing the lower unit (gearcase) to the engine's midsection and carefully lower it.
Remove the water pump housing cover to access the impeller assembly.
Slide the old impeller and washer(s) off the driveshaft, noting their order and orientation.
Clean the driveshaft and housing surfaces of any salt deposits, corrosion, or old gasket material.
Install the new washer onto the driveshaft in the correct position.
Reassemble the water pump with the new components, lower unit, and propeller.
Refill the gearcase with the manufacturer-specified marine gear lubricant.
Reconnect the battery, start the engine in a suitable water source, and verify strong water flow from the tell-tale outlet.

Troubleshooting & FAQ

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

Check the Engine & Model Compatibility table. It's crucial to match your engine's model, HP, and year range. Cross-reference your existing part number with the equivalents listed in the Part Number Matrix for the surest fit.

What are the signs of a failing impeller washer?

A worn, cracked, or corroded washer can compromise the seal within the water pump. This may lead to engine overheating, a weak or inconsistent water stream from the tell-tale, or accelerated wear of the impeller and housing.

Can I reuse my old washer during a water pump service?

It is strongly recommended to replace the washer and all other seals/gaskets every time you service the water pump or replace the impeller. A new washer is critical for ensuring a proper, leak-free seal.

How often should the water pump assembly, including this washer, be serviced?

As a best practice, inspect and service the entire water pump assembly (including the impeller, gaskets, and washer) every 100 hours of use or at least once per season, especially for boats operated in saltwater.

What is the difference between this part and an OEM one?

This is an aftermarket replacement part, not an original equipment manufacturer (OEM) component. It is engineered to meet or exceed OEM specifications for material, fit, and performance, offering a high-quality alternative at a better value.

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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