Sierra 18-7121 O-Ring, 3.100 Inch ID, Replaces Mercury 25-35029, for OMC and Mercruiser Stern Drives

The Sierra 18-7121 is a rubber O-ring built for OMC, Mercury, and Mercruiser outboards and stern drives. It has a 3.100 inch (78.7 mm) inside diameter and a 0.135 inch (3.4 mm) width, and is sold as one O-ring per package. The compound is black rubber, sized to seal against fluid loss and contamination at stern drive and outboard housing joints where a properly dimensioned O-ring is required for a tight fit. Sierra manufactures this part to meet or exceed the specifications of the original equipment part it replaces, making it a direct equivalent for OMC Stern Drive, Mercury Marine, and Mercruiser Stern Drive applications, as well as compatible with Mallory and GLM cross-reference numbers. Because O-rings harden and lose sealing ability with age and heat cycling, this is a common maintenance replacement item to keep on hand for stern drive service or rebuild work. Its simple, small-parts design means it installs by hand into the mating groove during reassembly, with no special tools required.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-7121
• Inside Diameter: 3.100 inches (78.7 mm)
• Width: 0.135 inches (3.4 mm)
• Material: Rubber
• Color: Black
• Unit Count: 1 O-ring per package
• Weight: 0.01 pounds
• UPC: 030999321398

Fitment

• OMC, Mercury, and Mercruiser outboards and stern drives

Replaces (OEM)

• Mallory 9-76651
• GLM 82350
• Mercury Marine 25-35029
• OMC Stern Drive 308621
• Mercruiser Stern Drive 25-35029
• Mercruiser Stern Drive 27-882613
• Mercruiser Stern Drive 802231

Frequently Asked Questions

What is the inside diameter of the Sierra 18-7121 O-ring? The Sierra 18-7121 has a 3.100 inch (78.7 mm) inside diameter and a 0.135 inch (3.4 mm) width.

Which stern drives and outboards does this O-ring fit? The 18-7121 is designed for use with OMC, Mercury, and Mercruiser outboards and stern drives.

Is the Sierra 18-7121 the same as OMC part 308621? Yes, the Sierra 18-7121 is interchangeable with OMC Stern Drive part number 308621, along with several other OEM equivalents.

What OEM part numbers does the 18-7121 replace? It replaces Mallory 9-76651, GLM 82350, Mercury Marine 25-35029, OMC Stern Drive 308621, and Mercruiser Stern Drive 25-35029, 27-882613, and 802231.

How many O-rings come in one package? Each package of the Sierra 18-7121 contains one O-ring.

What material is the Sierra 18-7121 O-ring made from? It is made from black rubber, sized for a durable seal in stern drive and outboard housing applications.

How much does the Sierra 18-7121 O-ring weigh? The Sierra 18-7121 O-ring weighs 0.01 pounds.

Is the Sierra 18-7121 a genuine OEM part or aftermarket? The 18-7121 is a Sierra-manufactured replacement part built to meet or exceed the original equipment part it replaces, rather than a factory OEM part.

Do I need any special tools to install the 18-7121 O-ring? No special tools are needed; the 18-7121 is a small O-ring that fits into its mating groove by hand during reassembly.

When should the 18-7121 O-ring be replaced? Because O-rings harden and lose their sealing ability over time and with heat exposure, the 18-7121 should be replaced any time the sealed joint is disassembled for service or if leakage is found.

What is the UPC for the Sierra 18-7121? The Sierra 18-7121 O-ring carries UPC 030999321398.