Mercury MerCruiser OEM Water Pump Base Oil Seal 79831 | 26-79831 | Replaces 26782 | Lower Unit Driveshaft Seal

Mercury MerCruiser OEM Water Pump Base Oil Seal — Part 79831

• Part Number: 79831
• Full Catalog Number: 26-79831
• Brand: Mercury Marine / MerCruiser (Genuine OEM)
• Official Product Type: Lower Unit — Water Pump Base Oil Seal (Driveshaft Seal)
• System: Lower unit / gearcase — water pump base and driveshaft housing sealing
• Position: Upper oil seal in the water pump base, sealing around the driveshaft
• Replaces: 26782 / 26-26782 (earlier Mercury oil seal)
• Commonly Paired With: Lower water pump base seal 90562
• Related Kit: 79831A1 / 26-79831A1 (seal kit for applicable gearcase assemblies)
• Weight: approx. 0.02 kg

Position matters: The 79831 is specifically the water pump base oil seal — it seals around the driveshaft beneath the water pump base, not the propeller shaft or upper drive input shaft. It is commonly installed alongside lower seal 90562 in the same water pump base assembly. If the lower unit is already apart for leakage diagnosis, inspect and replace both seals and the driveshaft sealing surface before reassembly.

What It Does

The 79831 / 26-79831 is the genuine OEM oil seal used at the water pump base position in a range of classic Mercury and MerCruiser lower-unit gearcase assemblies. It seals around the driveshaft beneath the water pump base, preventing gear lubricant from migrating up into the water pump cavity and keeping water out of the gear housing. Mercury parts diagrams identify 79831 as either the water pump base oil seal or the upper oil seal in the driveshaft housing, depending on the specific gearcase. It replaces earlier Mercury seal reference 26782 / 26-26782.

Compatible Applications

Confirm fitment by the exact Mercury or MerCruiser serial number and the specific parts diagram for the applicable lower-unit or gearcase before ordering. Mercury used several water-pump-base and driveshaft seal arrangements across different gearcase families and production ranges.

Part 79831 appears in water pump base assemblies including:

• Water pump base 77177A1
• Water pump base 77516A1
• Water pump base 73640T01
• Earlier water pump base 48745A1
• Earlier water pump base 65498A1
• Related classic Mercury and MerCruiser driveshaft housing assemblies

Not sure it fits? Send us your Mercury or MerCruiser serial number before ordering.

Related Seal & Water Pump Components

Mercury parts diagrams show 79831 installed with the following components. These are sold separately and are not included with the seal:

• 90562 — Lower water pump base oil seal (commonly replaced at the same service)
• 79831A1 / 26-79831A1 — Related Mercury seal kit for applicable gearcase assemblies
• 35973 — Water pump base dowel pins
• 19553 — Lower water pump gasket
• 32768 — Water pump face plate
• 19551 — Upper water pump gasket
• 89983T — Water pump impeller
• 8M0032837 — Impeller drive key
• 26855 / 26722 — Water pump base O-ring (application-dependent)

Part Numbers

• 79831 — Current Mercury Marine OEM part number
• 26-79831 — Full Mercury / Quicksilver catalog number
• 26782 / 26-26782 — Earlier Mercury oil seal; superseded by 79831
• 79831A1 / 26-79831A1 — Related Mercury seal kit for applicable gearcase assemblies

When to Replace

Replace the oil seal when:

• Gear lubricant is leaking into the water pump cavity
• Water is entering the gearcase at the driveshaft area
• A pressure or vacuum test shows leakage at the driveshaft seal location
• The water pump base is removed during lower-unit service
• The seal lip is hardened, torn, or damaged on inspection
• The driveshaft shows corrosion, grooving, or wear at the seal contact surface

Milky or water-contaminated gear oil indicates a breach in the gearcase sealing system. Pressure- and vacuum-test the lower unit to identify the seal location before ordering parts, and replace all seals disturbed during the repair.

Service Interval

No fixed hour-based replacement interval — replace on condition. Inspect the lower-unit oil seals whenever the water pump is serviced or the gear oil is changed and shows contamination. On older classic Mercury and MerCruiser gearcases, routine water pump service (impeller replacement) is a good opportunity to inspect 79831, 90562, and the driveshaft sealing surface for wear. Proactive seal replacement during water pump service avoids a second lower-unit disassembly if a seal fails shortly after.

Installation Notes

• Confirm 79831 is the correct water pump base oil seal for the specific gearcase serial number and parts diagram before installing
• Inspect the driveshaft sealing surface for grooves, corrosion, or wear before fitting a new seal — a damaged sealing surface will cause premature failure of the new seal
• Confirm correct seal orientation before pressing in — incorrect orientation will result in leakage
• Inspect companion lower seal 90562 while the water pump base is removed and replace it at the same time if there is any doubt about its condition
• Replace the water pump base O-ring (26855 or 26722, application-dependent) and any gaskets disturbed during disassembly
• Lubricate the seal lip with the lubricant specified by the applicable Mercury service procedure
• Pressure- and vacuum-test the lower unit after reassembly to confirm all seals are seating correctly before returning the drive to service

Technical FAQ

Q: What is the difference between the 79831 and the 79831A1?
79831 is the individual oil seal. 79831A1 / 26-79831A1 is a related Mercury seal kit used in applicable gearcase assemblies that may include additional seals or components for the same lower-unit repair. Confirm which reference is shown in the parts diagram for the specific gearcase being serviced.

Q: Should I replace 79831 and 90562 at the same time?
Yes, in most cases. When the water pump base is removed, both 79831 (upper) and 90562 (lower) seals are accessible. Replacing both at the same time avoids a second disassembly if the companion seal fails shortly after. Both seals seal the same driveshaft and are typically replaced together during lower-unit resealing work.

Q: My gear oil is milky — is 79831 the cause?
Milky gear oil indicates water intrusion into the gearcase, which can come from the water pump base seals, the propeller shaft seals, or the driveshaft housing area. A pressure and vacuum test on the lower unit will identify which seal location is leaking. 79831 is one possible source, but the propeller shaft seals and shift shaft seals should also be inspected before assuming 79831 is the only failure point.

Q: Does 79831 fit both Mercury outboards and MerCruiser sterndrive lower units?
79831 appears in multiple classic Mercury and MerCruiser gearcase families. Confirm the exact gearcase part number and serial number against the Mercury parts diagram before ordering — the seal appearance may be similar across different lower-unit families, but fitment must be confirmed by the specific diagram.

Parts Desk Insights

The most important detail with 79831 is its position — it is the water pump base oil seal around the driveshaft, not the propeller shaft seal or the upper input shaft seal. Confirming the exact diagram location prevents ordering the wrong seal. In practice, when a customer is dealing with gear oil leakage at the water pump area, the companion lower seal 90562 should also be on the order — both are accessible during the same disassembly, and replacing only one while leaving an aged companion seal in place is a common reason for a repeat lower-unit repair within a season.