{"product_id":"sierra-18-3902-marine-piston-ring-standard-2-875-in-bore-replaces-mercury-mariner-oem-39-78816a12-35-140-hp-outboards","title":"Sierra 18-3902 Piston Ring Set, Replaces Mercury 39-78816A12, 2.875 Inch Standard Bore, 35-140 HP 2-6 Cylinder Outboards","description":"\u003cp\u003eThe 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.\u003c\/p\u003e\u003cp\u003eThis 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.\u003c\/p\u003e\u003ch3\u003eSpecifications\u003c\/h3\u003e\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eBrand:\u003c\/strong\u003e Sierra\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eManufacturer Part Number:\u003c\/strong\u003e 18-3902\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eOEM Replaces:\u003c\/strong\u003e 39-78816A12\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eRing Type:\u003c\/strong\u003e 3 Ring, Low Dome Piston\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eBore Size:\u003c\/strong\u003e 2.875 inches, Standard\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eHorsepower Range:\u003c\/strong\u003e 35-140 HP\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eCylinder Configuration:\u003c\/strong\u003e 2 to 6 cylinders\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eRings Required per Piston:\u003c\/strong\u003e 4\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eProduct Dimensions:\u003c\/strong\u003e 0.39 x 0.39 x 0.39 inches\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eWeight:\u003c\/strong\u003e 0.06 pounds\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eUPC:\u003c\/strong\u003e 030999316967\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch3\u003eFitment\u003c\/h3\u003e\u003cul\u003e\n\u003cli\u003eMercury outboards, 35-140 HP\u003c\/li\u003e\n\u003cli\u003e2 to 6 cylinder engines\u003c\/li\u003e\n\u003cli\u003e2.875 inch standard bore, low-dome 3-ring piston\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch3\u003eReplaces (OEM)\u003c\/h3\u003e\u003cul\u003e\u003cli\u003e39-78816A12\u003c\/li\u003e\u003c\/ul\u003e\u003ch3\u003eFrequently Asked Questions\u003c\/h3\u003e\u003cp\u003e\u003cstrong\u003eWhat does Sierra 18-3902 replace?\u003c\/strong\u003e Sierra 18-3902 is a direct replacement for OEM Mercury Marine piston ring number 39-78816A12.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat Mercury outboards does this piston ring fit?\u003c\/strong\u003e 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.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eHow many 18-3902 rings do I need per piston?\u003c\/strong\u003e Four units of 18-3902 are required to complete one full piston ring set equivalent to OEM Mercury 39-78816A12.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eIs this ring for a standard or oversized bore?\u003c\/strong\u003e 18-3902 is sized for a 2.875 inch standard bore, not an oversized bore.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eIs the 18-3902 a 2-ring or 3-ring design?\u003c\/strong\u003e It is a 3-ring design intended for the low-dome piston application in this bore and HP range.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eDoes this ring come from an authorized Mercury parts supplier?\u003c\/strong\u003e Sierra 18-3902 is manufactured by Sierra as an aftermarket equivalent to OEM Mercury part 39-78816A12.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat size is the 18-3902 piston ring?\u003c\/strong\u003e Product dimensions for the Sierra 18-3902 are 0.39 x 0.39 x 0.39 inches, weighing 0.06 pounds.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eCan I use 18-3902 on a 2-cylinder Mercury outboard?\u003c\/strong\u003e Yes, 18-3902 covers 2 to 6 cylinder Mercury outboards within the 35-140 HP range at the 2.875 inch standard bore.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat is included when I order 18-3902?\u003c\/strong\u003e Each 18-3902 order is for an individual piston ring; four are needed to match one OEM 39-78816A12 set on a single piston.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eHow do I know if I need the standard bore version or an oversized version?\u003c\/strong\u003e 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.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat is the OEM cross-reference for Sierra 18-3902?\u003c\/strong\u003e The OEM cross-reference for Sierra 18-3902 is Mercury Marine 39-78816A12.\u003c\/p\u003e","brand":"Sierra","offers":[{"title":"Default Title","offer_id":48988169896155,"sku":"SIE-18-3902","price":41.37,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0839\/7586\/4539\/files\/18-3902__09663.1722373858.500.750.jpg?v=1785188759","url":"https:\/\/www.goodvibesmarina.com\/products\/sierra-18-3902-marine-piston-ring-standard-2-875-in-bore-replaces-mercury-mariner-oem-39-78816a12-35-140-hp-outboards","provider":"Good Vibes Marina","version":"1.0","type":"link"}