Sierra 18-3902 Piston Ring Set, Replaces Mercury 39-78816A12, 2.875 Inch Standard Bore, 35-140 HP 2-6 Cylinder Outboards

The Sierra 18-3902 is a piston ring for Mercury outboard motors, engineered for the 3-ring, low-dome piston application at 2.875 inch standard bore. It is a direct replacement for OEM Mercury Marine piston ring number 39-78816A12, giving outboard owners and marine technicians a reliable source for maintaining proper cylinder sealing, compression, and oil control during a piston or ring-set rebuild. Four rings (four units of 18-3902) are required to complete one full piston ring set equivalent to the OEM 39-78816A12 grouping, so buyers should order accordingly based on the number of pistons being serviced.

This ring set covers Mercury outboards in the 35-140 HP range with 2 to 6 cylinder configurations, matched to the 2.875 inch standard bore, low-dome 3-ring piston. It is intended for use during a piston and ring replacement job as part of a powerhead rebuild, helping restore compression and reduce blow-by in engines with worn or scored original rings. As with any precision engine component, correct bore size and piston type should be confirmed before ordering to ensure proper fit.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3902
• OEM Replaces: 39-78816A12
• Ring Type: 3 Ring, Low Dome Piston
• Bore Size: 2.875 inches, Standard
• Horsepower Range: 35-140 HP
• Cylinder Configuration: 2 to 6 cylinders
• Rings Required per Piston: 4
• Product Dimensions: 0.39 x 0.39 x 0.39 inches
• Weight: 0.06 pounds
• UPC: 030999316967

Fitment

• Mercury outboards, 35-140 HP
• 2 to 6 cylinder engines
• 2.875 inch standard bore, low-dome 3-ring piston

Replaces (OEM)

• 39-78816A12

Frequently Asked Questions

What does Sierra 18-3902 replace? Sierra 18-3902 is a direct replacement for OEM Mercury Marine piston ring number 39-78816A12.

What Mercury outboards does this piston ring fit? The 18-3902 ring fits Mercury outboards in the 35-140 HP range with 2 to 6 cylinder configurations, matched to a 2.875 inch standard bore, low-dome 3-ring piston.

How many 18-3902 rings do I need per piston? Four units of 18-3902 are required to complete one full piston ring set equivalent to OEM Mercury 39-78816A12.

Is this ring for a standard or oversized bore? 18-3902 is sized for a 2.875 inch standard bore, not an oversized bore.

Is the 18-3902 a 2-ring or 3-ring design? It is a 3-ring design intended for the low-dome piston application in this bore and HP range.

Does this ring come from an authorized Mercury parts supplier? Sierra 18-3902 is manufactured by Sierra as an aftermarket equivalent to OEM Mercury part 39-78816A12.

What size is the 18-3902 piston ring? Product dimensions for the Sierra 18-3902 are 0.39 x 0.39 x 0.39 inches, weighing 0.06 pounds.

Can I use 18-3902 on a 2-cylinder Mercury outboard? Yes, 18-3902 covers 2 to 6 cylinder Mercury outboards within the 35-140 HP range at the 2.875 inch standard bore.

What is included when I order 18-3902? Each 18-3902 order is for an individual piston ring; four are needed to match one OEM 39-78816A12 set on a single piston.

How do I know if I need the standard bore version or an oversized version? Measure the cylinder bore before ordering; 18-3902 is specifically the 2.875 inch standard bore, low-dome 3-ring version, and oversized bores require a different part number.

What is the OEM cross-reference for Sierra 18-3902? The OEM cross-reference for Sierra 18-3902 is Mercury Marine 39-78816A12.