{"title":"GM 4-Stroke 120HP Marine Engine Parts","description":"\u003csection id=\"hero\" style=\"padding:16px; border:1px solid #e5e7eb; border-radius:8px;\"\u003e\n  \u003cp\u003eThis page is dedicated to GM 4-Stroke 120HP marine engines based on the 2.5L (153 CID) inline-4 platform commonly used in MerCruiser and OMC sterndrive packages. Use it to find compatible parts and practical service information. Below you’ll find maintenance tips, common replacement parts, and troubleshooting guidance. You can also browse the main MerCruiser collection for a broader range of components. Scroll down to the bottom of this page to start browsing parts for this engine, and on the collection page use the Filter \u0026amp; Sort controls to narrow by horsepower, year range, and part category.\u003c\/p\u003e\n  \u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser\" style=\"display:inline-block; padding:10px 16px; background:#0d6efd; color:#fff; text-decoration:none; border-radius:6px;\"\u003eBrowse All MerCruiser Parts\u003c\/a\u003e\n\u003c\/section\u003e\n\n\u003csection id=\"identify-model\" style=\"padding:16px;\"\u003e\n  \u003ch2\u003eHow to Identify Your 120HP Model\u003c\/h2\u003e\n  \u003cul\u003e\n    \u003cli\u003eNameplate\/serial location: check the sterndrive transom assembly tag and the engine block pad (starboard side) for stamped serials or casting numbers.\u003c\/li\u003e\n    \u003cli\u003eModel\/serial decoding: MerCruiser uses a serial number sequence on the engine and drive; OMC tags indicate Stringer or Cobra drive with year-coded suffixes. Match parts by engine and drive serials.\u003c\/li\u003e\n    \u003cli\u003ePlatform distinctions: This 120HP package uses the GM 2.5L inline-4. External marinization parts differ between MerCruiser and OMC (exhaust manifolds\/elbows, ignition, brackets, raw-water pump location).\u003c\/li\u003e\n    \u003cli\u003eDrive type cues: Pre-Alpha\/Alpha One drives have the raw-water pump in the lower unit; some OMC packages use an engine-mounted pump. Note this when selecting impellers and pump kits.\u003c\/li\u003e\n  \u003c\/ul\u003e\n\u003c\/section\u003e\n\n\u003csection id=\"engine-overview\" style=\"padding:16px;\"\u003e\n  \u003ch2\u003eEngine Overview\u003c\/h2\u003e\n  \u003ch3\u003eGM 2.5L (153 CID) Inline-4 – 120 HP\u003c\/h3\u003e\n  \u003ctable style=\"width:100%; border-collapse:collapse;\"\u003e\n    \u003cthead\u003e\n      \u003ctr\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eFeature\u003c\/th\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eDescription\u003c\/th\u003e\n      \u003c\/tr\u003e\n    \u003c\/thead\u003e\n    \u003ctbody\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eConfiguration\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eInline-4, carbureted 4-stroke, GM 2.5L (153 CID) base; typically 1-bbl carburetor with breaker-point or electronic ignition depending on year\/package.\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eCommon Models\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eMerCruiser 120; OMC 120 (GM 2.5L). Serial-driven differences affect ignition, manifolds, and cooling hardware.\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eAdvantages\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eSimple, reliable, and well-supported; broad parts availability; economical fuel burn at cruise.\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px;\"\u003eNotes\u003c\/td\u003e\n        \u003ctd style=\"padding:8px;\"\u003eThe base long block is shared, but marinization components (exhaust, ignition, mounts, water pump location) vary by brand and drive. Always confirm by engine and drive serial numbers before ordering.\u003c\/td\u003e\n      \u003c\/tr\u003e\n    \u003c\/tbody\u003e\n  \u003c\/table\u003e\n\u003c\/section\u003e\n\n\u003csection id=\"service-kits\" style=\"padding:16px;\"\u003e\n  \u003ch2\u003eRoutine Maintenance \u0026amp; Service Parts\u003c\/h2\u003e\n  \u003cp\u003eIntervals below are general guidelines for typical recreational use. Always follow the official service manual for your specific engine and drive.\u003c\/p\u003e\n  \u003ctable style=\"width:100%; border-collapse:collapse;\"\u003e\n    \u003cthead\u003e\n      \u003ctr\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eItem\u003c\/th\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eInterval*\u003c\/th\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eRemarks\u003c\/th\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eParts Link\u003c\/th\u003e\n      \u003c\/tr\u003e\n    \u003c\/thead\u003e\n    \u003ctbody\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eEngine oil + filter\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e100 hours or annually\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eUse FC-W marine oil (e.g., 25W-40). Warm engine before draining; replace gasket and pre-fill filter as applicable.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e\u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-oil-filters\"\u003eMerCruiser Oil Filters\u003c\/a\u003e\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eGear oil (sterndrive)\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e100 hours or annually\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eDrain from lower plug; check for water\/milkiness and metal on the magnet. Refill from the bottom until it appears at the vent.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e\u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser\"\u003eMerCruiser\u003c\/a\u003e\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eSpark plugs\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e100 hours or annually\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eReplace and gap to spec; inspect leads and cap\/rotor (where fitted).\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e\u003ca href=\"https:\/\/seasierra.com\/collections\/spark-plug\"\u003eSpark plug\u003c\/a\u003e\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eFuel filter \/ water separator\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e100 hours or annually\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eDrain bowl (if equipped); replace element; check for debris or water.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e\u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-fuel-filters\"\u003eMerCruiser Fuel Filters\u003c\/a\u003e\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eWater pump impeller\/kit\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e2 seasons or 200 hours (sooner in sandy\/silty water)\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eAlpha-style drives use a lower-unit impeller; some OMC packages use an engine-mounted pump—match by drive\/serial.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e\u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-impellers\"\u003eMerCruiser Impellers\u003c\/a\u003e\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eAnodes (drive \u0026amp; trim tabs)\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eInspect monthly; replace at ~50% depletion\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eUse the correct alloy (zinc\/salt, aluminum\/brackish, magnesium\/fresh). Clean contact surfaces.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e\u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-anodes\"\u003eMerCruiser Anodes\u003c\/a\u003e\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eDrive bellows (exhaust\/U-joint\/shift)\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eInspect annually; replace every 2–3 seasons\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eCracked or loose bellows can let water in, damaging U-joints and gimbal bearing.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003e\u003ca href=\"https:\/\/seasierra.com\/collections\/bellow\"\u003eBellow\u003c\/a\u003e\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px;\"\u003eBelts (alternator\/PS)\u003c\/td\u003e\n        \u003ctd style=\"padding:8px;\"\u003eInspect each trip; replace if cracked or glazed\u003c\/td\u003e\n        \u003ctd style=\"padding:8px;\"\u003eSet correct tension; re-check after first hour on new belts.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px;\"\u003e\u003ca href=\"https:\/\/seasierra.com\/collections\/belt\"\u003eBelt\u003c\/a\u003e\u003c\/td\u003e\n      \u003c\/tr\u003e\n    \u003c\/tbody\u003e\n  \u003c\/table\u003e\n\u003c\/section\u003e\n\n\u003csection id=\"common-parts\" style=\"padding:16px;\"\u003e\n  \u003ch2\u003eFrequently Used Parts Categories\u003c\/h2\u003e\n  \u003cul\u003e\n    \u003cli\u003eRoutine Service – \u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-service-maintenance-kits\"\u003eMercruiser Service \u0026amp; Maintenance Kits\u003c\/a\u003e\n\u003c\/li\u003e\n    \u003cli\u003eCooling System – \u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-impellers\"\u003eMerCruiser Impellers\u003c\/a\u003e\n\u003c\/li\u003e\n    \u003cli\u003eFuel System – \u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-fuel-filters\"\u003eMerCruiser Fuel Filters\u003c\/a\u003e\n\u003c\/li\u003e\n    \u003cli\u003eIgnition \/ Electrical – \u003ca href=\"https:\/\/seasierra.com\/collections\/electrical-items\"\u003eElectrical Items\u003c\/a\u003e\n\u003c\/li\u003e\n    \u003cli\u003eExhaust \u0026amp; Manifolds – \u003ca href=\"https:\/\/seasierra.com\/collections\/manifolds\"\u003eManifolds\u003c\/a\u003e\n\u003c\/li\u003e\n    \u003cli\u003eCorrosion Protection – \u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-anodes\"\u003eMerCruiser Anodes\u003c\/a\u003e\n\u003c\/li\u003e\n    \u003cli\u003ePropellers \u0026amp; Hardware – \u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser-propellers\"\u003eMerCruiser Propellers\u003c\/a\u003e\n\u003c\/li\u003e\n  \u003c\/ul\u003e\n\u003c\/section\u003e\n\n\u003csection id=\"troubleshooting\" style=\"padding:16px;\"\u003e\n  \u003ch2\u003eCommon Symptoms \u0026amp; Quick Checks\u003c\/h2\u003e\n  \u003ctable style=\"width:100%; border-collapse:collapse;\"\u003e\n    \u003cthead\u003e\n      \u003ctr\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eSymptom\u003c\/th\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eCheck First\u003c\/th\u003e\n        \u003cth style=\"text-align:left; border-bottom:1px solid #e5e7eb; padding:8px;\"\u003eRelated Parts\u003c\/th\u003e\n      \u003c\/tr\u003e\n    \u003c\/thead\u003e\n    \u003ctbody\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eOverheating at idle or high RPM\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eInspect raw-water impeller condition, inlet screens for debris, and thermostat operation. Check hoses for collapse and air leaks.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eWater pump impeller kit, pump housing\/gaskets, thermostat, hoses\/clamps.\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003ePoor acceleration \/ power loss\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eVerify fuel quality, replace fuel filter, check propeller for damage or hub slip, confirm ignition timing\/advance and plug condition.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eFuel filter, fuel pump, spark plugs\/ignition parts, propeller.\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eHard starting \/ rough idle\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eCheck choke\/enrichment, clean carburetor and idle circuits, look for vacuum leaks, test battery and starter draw.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px; border-bottom:1px solid #f0f0f0;\"\u003eCarburetor service kit, spark plugs, gaskets\/hoses, starter motor.\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd style=\"padding:8px;\"\u003eGearcase noise or milky gear oil\u003c\/td\u003e\n        \u003ctd style=\"padding:8px;\"\u003eConfirm gear lube level\/condition, inspect prop shaft and input shaft seals, check bellows and gimbal bearing; verify shift cable adjustment.\u003c\/td\u003e\n        \u003ctd style=\"padding:8px;\"\u003eGear lube, lower unit seal kit, bellows, gimbal bearing, shift cable.\u003c\/td\u003e\n      \u003c\/tr\u003e\n    \u003c\/tbody\u003e\n  \u003c\/table\u003e\n\u003c\/section\u003e\n\n\u003csection id=\"faq\" style=\"padding:16px;\"\u003e\n  \u003ch2\u003eFAQs\u003c\/h2\u003e\n  \u003cdetails\u003e\n    \u003csummary\u003eAre parts interchangeable between MerCruiser and OMC 120HP packages?\u003c\/summary\u003e\n    \u003cp\u003eThe base GM 2.5L long block is shared, but marinization parts (exhaust manifolds\/elbows, ignition, raw-water pump, brackets) are brand- and year-specific. Always match parts by engine and drive serial numbers.\u003c\/p\u003e\n  \u003c\/details\u003e\n  \u003cdetails\u003e\n    \u003csummary\u003eWhen should the raw-water impeller be replaced?\u003c\/summary\u003e\n    \u003cp\u003eAs a preventive measure, replace every 2 seasons or ~200 hours—sooner if you run in sandy\/silty water or notice higher temperatures or weak cooling flow. Inspect the wear plate and cup when servicing.\u003c\/p\u003e\n  \u003c\/details\u003e\n  \u003cdetails\u003e\n    \u003csummary\u003eHow often should sterndrive gear oil be changed?\u003c\/summary\u003e\n    \u003cp\u003eChange at least once per season or every 100 hours. If water (milky oil) or excessive metal is found, stop using the boat and address seals, bellows, or bearing issues before further operation.\u003c\/p\u003e\n  \u003c\/details\u003e\n  \u003cdetails\u003e\n    \u003csummary\u003eWhat information do you need to confirm part compatibility?\u003c\/summary\u003e\n    \u003cp\u003eProvide the engine serial, drive serial (Alpha\/Pre-Alpha or OMC type), carburetor\/ignition details, and any casting or part numbers. Clear photos of the nameplate and current parts also help ensure a correct match.\u003c\/p\u003e\n  \u003c\/details\u003e\n\u003c\/section\u003e\n\n\u003csection id=\"cta\" style=\"padding:16px; border-top:1px solid #e5e7eb;\"\u003e\n  \u003cp\u003eReady to find the right components? Browse the main MerCruiser collection and use Filter \u0026amp; Sort to narrow by horsepower, year, and model.\u003c\/p\u003e\n  \u003ca href=\"https:\/\/seasierra.com\/collections\/mercruiser\" style=\"display:inline-block; padding:10px 16px; background:#0d6efd; color:#fff; text-decoration:none; border-radius:6px;\"\u003eShop MerCruiser Parts\u003c\/a\u003e\n  \u003cp style=\"margin-top:8px; color:#555;\"\u003eIf you’re unsure about your model, send us a photo of the nameplate and the part you need and we’ll help verify compatibility.\u003c\/p\u003e\n\u003c\/section\u003e","products":[],"url":"https:\/\/seasierra.com\/collections\/gm-4-stroke-120hp-marine-engine-parts.oembed","provider":"SeaSierra","version":"1.0","type":"link"}