Mercury MerCruiser End Cap O-Ring 25-35126 | 65–300 HP Applications

Mercury MerCruiser End Cap O-Ring — Part No. 25-35126

This is the genuine Mercury Marine OEM end cap O-ring listed under part number 25-35126 (base number 35126). Used on select Mercury outboard and MerCruiser sterndrive applications in the 65–300 HP range, this O-ring seals the end cap assembly at the location specified in the Mercury parts diagram. Like all OEM Mercury O-rings, it is compounded and sized for the specific sealing environment at this location — not a generic substitute.

Part Details

• Part Number: 25-35126
• Base Number: 35126
• Cross-Reference: 526-15
• Type: End Cap O-Ring
• Application Range: Select Mercury / MerCruiser 65–300 HP applications
• Type: Genuine Mercury Marine OEM replacement O-ring

Compatible Applications

• Select Mercury outboard engines — 65–300 HP range, where 25-35126 is shown in the parts diagram
• Select MerCruiser sterndrive applications — where 25-35126 is shown in the parts diagram

Confirm fitment by engine model, horsepower, serial number, and parts diagram before ordering. Mercury fitment can vary by model year, assembly version, and production range within the 65–300 HP range.

Alternate, Superseded & Cross-Reference Part Numbers

• 25-35126 — current Mercury/MerCruiser formatted number
• 35126 — base part number
• 526-15 — alternate cross-reference

When to Replace

Replace this O-ring any time the end cap is removed for service. O-rings that have been compressed, heat-cycled, or exposed to fuel or oil should not be reused — even if they appear undamaged. Also replace if the original seal is flattened, brittle, cracked, swollen, cut, or leaking.

Installation Tips

• Clean the end cap O-ring groove and mating surface before installation — debris or corrosion in the groove will prevent a proper seal
• Lightly lubricate the O-ring with the appropriate Mercury-approved lubricant before installation to prevent tearing during assembly
• Install the O-ring evenly without twisting or pinching — a twisted O-ring will not seal correctly
• Confirm the O-ring is fully seated in the groove before assembling the end cap
• Never reuse the original O-ring after the end cap has been removed

Technical FAQ

Q: How do I confirm this is the right O-ring for my engine?
A: Look up your engine model and serial number in the Mercury parts diagram and confirm the diagram calls for 25-35126 or 35126 at the end cap location you are servicing. The cross-reference 526-15 may also appear in older catalog references for the same part.

Q: Can I use a generic O-ring of the same size?
A: Not recommended. Mercury OEM O-rings are compounded for compatibility with the specific fluids, temperatures, and pressures at the sealing location. A generic substitute may degrade faster or not seal correctly in a marine environment.

Q: Why does the HP range span 65–300 HP?
A: This O-ring is used across a broad range of Mercury outboard and MerCruiser applications where the end cap assembly uses the same sealing specification. The exact application must be confirmed by serial number and diagram — not by horsepower alone.

Parts Desk Insights

Best use case: This O-ring is the right call when the Mercury parts diagram specifically calls for 25-35126 or its cross-reference 526-15 at the end cap location. It’s a low-cost part that should be ordered ahead of any end cap service — having it on hand before teardown avoids delays waiting for a seal to arrive mid-job. The broad 65–300 HP application range means it appears across many common Mercury outboard and MerCruiser service jobs.

Common issues: The most frequent problem is searching only by the base number 35126 and missing the correct part context — using the full formatted number 25-35126 or the cross-reference 526-15 returns more reliable results. The second common issue is reusing the original O-ring after end cap removal. A seal that has been compressed and heat-cycled may look serviceable but will often leak on reassembly. Replace it every time the end cap comes off.