Oil Seal for Mercury 8 HP 4-Stroke Outboards – Replaces 26-413651 / Sierra 18-0581

Product Details

This oil seal is an aftermarket replacement for the driveshaft gear-housing application used on selected Mercury 8 HP 4-stroke outboard motors. Catalog information identifies the 8 HP 4-stroke, 209 cc configuration with serial range 0P325500 and up. The seal is intended to help retain gearcase lubricant and reduce the risk of leakage around the driveshaft sealing area.

This replacement is cross-referenced with Mercury Marine OEM part number 26-413651 and Sierra part number 18-0581. The Sierra catalog also lists 18-0581 as an oil seal associated with OEM number 26-413651.

• Oil seal for the lower-unit driveshaft gear-housing application
• Catalog-listed for selected Mercury 8 HP 4-stroke, 209 cc outboards
• Applicable catalog serial range: 0P325500 and up
• Aftermarket replacement for Mercury Marine OEM reference 26-413651

Part Number Matrix

Use these references to compare the replacement with the part number shown in your service information or on the original component.

Engine & Model Compatibility

Verify the complete model, displacement, serial number, and original part number before ordering. The catalog occurrence available for this part identifies the following application; other Mercury models or serial ranges are not confirmed here.

Technical Specs

Part Type

Oil Seal

Application

Lower-unit driveshaft gear housing

Compatible Engine

Mercury 8 HP 4-Stroke, 209 cc configuration listed in the catalog

Applicable Serial Range

0P325500 and up, subject to model and configuration verification

Mercury OEM Reference

26-413651

Aftermarket Reference

Sierra 18-0581

What's in the Box

• 1 × Oil Seal

Installation Notes

1. Turn the engine off, disconnect the battery where applicable, and secure the outboard before beginning work.
2. Remove the lower unit and access the driveshaft gear-housing sealing area according to the manufacturer's service manual.
3. Remove the old seal with a suitable seal puller or pick tool, taking care not to damage the bore, shaft, or housing.
4. Clean the seal bore and inspect the shaft and surrounding surfaces for wear, corrosion, or damage.
5. Apply only the lubricant or installation compound specified by the service manual to the seal lip and sealing surfaces.
6. Press the new seal into position squarely with a suitable seal driver or correctly sized installation tool.
7. Make sure the seal is seated to the manufacturer's specified depth; do not strike the sealing lip directly.
8. Reassemble the lower unit, refill or check the gearcase lubricant as specified, and inspect for leaks after installation.

Troubleshooting & FAQ

What are common signs of a failed lower-unit oil seal?

Possible signs include lubricant leakage near the gear housing or driveshaft area, a low gearcase lubricant level, or lubricant contaminated with water. Inspect the shaft, housing, and other seals before replacing a component.

Is this seal compatible with all Mercury 8 HP, 9.9 HP, and 15 HP engines?

No. The available catalog data confirms a Mercury 8 HP 4-stroke, 209 cc application with serial range 0P325500 and up. The 9.9 HP and 15 HP applications in the previous listing are not confirmed by the available part-specific catalog occurrence. Verify the exact model, displacement, serial number, and part number before purchase.

Does this listing contain a complete lower-unit seal kit?

No. This listing is for one oil seal only. It does not include the additional seals, washers, O-rings, water-tube seal, or water-pump base found in a complete lower-unit seal kit.

Can I reuse the old oil seal?

Oil seals should normally be replaced rather than reused after removal. A removed seal may be distorted or damaged and may not reseal reliably.

Do I need special tools to install the seal?

A suitable seal puller may help with removal, and a seal driver or correctly sized socket can help seat the replacement evenly. Follow the manufacturer's service procedure for your specific lower unit.

Is this an OEM Mercury part?

No. This is an aftermarket replacement. Mercury Marine OEM part number 26-413651 and Sierra aftermarket part number 18-0581 are provided as reference numbers for identification and cross-checking.

Safety & Important Info

• Confirm the full engine model, displacement, serial number, and original part number before ordering.
• Do not rely on horsepower alone to determine compatibility; gearcase design and serial range must also match.
• This is an aftermarket replacement, not an OEM Mercury component.
• Professional installation is recommended when lower-unit disassembly or seal replacement is outside your experience.

About Sea Sierra

• We supply replacement marine parts for repair and maintenance applications.
• Each listing is organized around available part-number and application information to support careful fitment checking.
• For best results, compare the original component and service-manual information before installation.