{"product_id":"thermostat-gasket-kit-fits-mercruiser-2-5l-120hp-3-0l-140hp-replaces-59078t3-59078a1-59078q3-59078b-59078","title":"Thermostat \u0026 Gasket Kit – Fits MerCruiser 2.5L 120HP \u0026 3.0L 140HP (Replaces 59078T3, 59078A1, 59078Q3, 59078B, 59078)","description":"\u003csection\u003e\n\u003c!-- 🔧 Table Styles (include only once per page) --\u003e\n\u003cstyle\u003e\n  \/* Outer container allows horizontal scrolling (with iOS momentum) *\/\n  .ss-table-wrap { overflow-x:auto; -webkit-overflow-scrolling:touch; }\n\n  \/* Base table styles *\/\n  table.ss-table { width:100%; border-collapse:collapse; min-width:640px; }\n  table.ss-table th, table.ss-table td {\n    padding:10px 12px; border:1px solid #e5e7eb; text-align:left; vertical-align:top;\n  }\n\n  \/* Key: prevent wrapping, word breaks, and vertical text in headers *\/\n  table.ss-table th {\n    white-space:nowrap;       \/* Do not wrap *\/\n    word-break:normal;        \/* Don’t break words *\/\n    letter-spacing:normal;    \/* Prevent theme styles from stretching letters *\/\n    font-weight:600;\n    background:#f8fafc;\n  }\n\n  \/* Small screen optimization *\/\n  @media (max-width:480px){\n    table.ss-table th, table.ss-table td { padding:8px 10px; font-size:14px; }\n  }\n\u003c\/style\u003e\n\n\u003ch2\u003eProduct Details\u003c\/h2\u003e\n\u003cp\u003eThis replacement \u003cstrong\u003eThermostat \u0026amp; Gasket Kit – Fits MerCruiser 2.5L 120HP \u0026amp; 3.0L 140HP (Replaces 59078T3)\u003c\/strong\u003e is designed for 4-cylinder MerCruiser 120HP (2.5L) and 140HP (3.0L) inboard\/sterndrive engines manufactured between 1968 and 2000. It ensures your engine maintains the correct operating temperature for optimal performance and longevity.\u003c\/p\u003e\n\u003cp\u003eThis kit serves as a direct replacement for Mercury\/MerCruiser part numbers 59078T3, 59078A1, 59078Q3, 59078B, and 59078. It also interchanges with aftermarket parts from Sierra (18-3650), Mallory Marine (9-43153), and OBR Red Rhino (MR-524).\u003c\/p\u003e\n\u003cul\u003e\n    \u003cli\u003eEngineered for direct-fit installation, no modifications required.\u003c\/li\u003e\n    \u003cli\u003eMaintains a stable engine operating temperature of 140°F (60°C).\u003c\/li\u003e\n    \u003cli\u003eComplete kit includes both the thermostat and the necessary housing gasket.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003ePart Number Matrix\u003c\/h2\u003e\n\u003cp\u003eUse this matrix to confirm equivalent references across OEM and aftermarket.\u003c\/p\u003e\n\u003cdiv class=\"ss-table-wrap\"\u003e\n  \u003ctable class=\"ss-table\" aria-label=\"Part Number Matrix\"\u003e\n    \u003cthead\u003e\n      \u003ctr\u003e\n        \u003cth\u003eBrand\u003c\/th\u003e\n        \u003cth\u003ePart Number\u003c\/th\u003e\n      \u003c\/tr\u003e\n    \u003c\/thead\u003e\n    \u003ctbody\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMercury\/MerCruiser\u003c\/td\u003e\n        \u003ctd\u003e59078T3\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMercury\/MerCruiser\u003c\/td\u003e\n        \u003ctd\u003e59078A1\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMercury\/MerCruiser\u003c\/td\u003e\n        \u003ctd\u003e59078Q3\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMercury\/MerCruiser\u003c\/td\u003e\n        \u003ctd\u003e59078B\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMercury\/MerCruiser\u003c\/td\u003e\n        \u003ctd\u003e59078\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eSierra\u003c\/td\u003e\n        \u003ctd\u003e18-3650\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eOBR Red Rhino\u003c\/td\u003e\n        \u003ctd\u003eMR-524\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMallory Marine\u003c\/td\u003e\n        \u003ctd\u003e9-43153\u003c\/td\u003e\n      \u003c\/tr\u003e\n    \u003c\/tbody\u003e\n  \u003c\/table\u003e\n\u003c\/div\u003e\n\n\u003ch2\u003eEngine \u0026amp; Model Compatibility\u003c\/h2\u003e\n\u003cp\u003eConfirm your engine model\/year in the table below.\u003c\/p\u003e\n\u003cp\u003eThis thermostat kit fits a wide range of MerCruiser engines, including specific 2.5L, 3.0L, and inline 6-cylinder models. Verify your engine's serial number falls within the specified ranges for an exact match.\u003c\/p\u003e\n\u003cdiv class=\"ss-table-wrap\"\u003e\n  \u003ctable class=\"ss-table\" aria-label=\"Engine \u0026amp; Model Compatibility\"\u003e\n    \u003cthead\u003e\n      \u003ctr\u003e\n        \u003cth\u003eBrand\u003c\/th\u003e\n        \u003cth\u003eModel\u003c\/th\u003e\n        \u003cth\u003eRated HP\u003c\/th\u003e\n        \u003cth\u003eStroke\u003c\/th\u003e\n        \u003cth\u003eYears\u003c\/th\u003e\n        \u003cth\u003eOEM Part No.\u003c\/th\u003e\n        \u003cth\u003eNotes\u003c\/th\u003e\n      \u003c\/tr\u003e\n    \u003c\/thead\u003e\n    \u003ctbody\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMerCruiser\u003c\/td\u003e\n        \u003ctd\u003e120 - 2.5L \/ L4 - 153\u003c\/td\u003e\n        \u003ctd\u003e120\u003c\/td\u003e\n        \u003ctd\u003e4-Stroke\u003c\/td\u003e\n        \u003ctd\u003e1968–1989\u003c\/td\u003e\n        \u003ctd\u003e59078T3\u003c\/td\u003e\n        \u003ctd\u003eS\/N 2324347 - 0C853255\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMerCruiser\u003c\/td\u003e\n        \u003ctd\u003e140 - 3.0L \/ L4 - 181\u003c\/td\u003e\n        \u003ctd\u003e140\u003c\/td\u003e\n        \u003ctd\u003e4-Stroke\u003c\/td\u003e\n        \u003ctd\u003e1968–1989\u003c\/td\u003e\n        \u003ctd\u003e59078T3\u003c\/td\u003e\n        \u003ctd\u003eS\/N 2508261 - 0C856558\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMerCruiser\u003c\/td\u003e\n        \u003ctd\u003e3.0L \/ L4 - 181\u003c\/td\u003e\n        \u003ctd\u003e140\u003c\/td\u003e\n        \u003ctd\u003e4-Stroke\u003c\/td\u003e\n        \u003ctd\u003e1990–1994\u003c\/td\u003e\n        \u003ctd\u003e59078T3\u003c\/td\u003e\n        \u003ctd\u003eS\/N 0C856559 - 0F603999\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMerCruiser\u003c\/td\u003e\n        \u003ctd\u003e3.0L \/ L4 - 181\u003c\/td\u003e\n        \u003ctd\u003e140\u003c\/td\u003e\n        \u003ctd\u003e4-Stroke\u003c\/td\u003e\n        \u003ctd\u003e1998–2000\u003c\/td\u003e\n        \u003ctd\u003e59078T3\u003c\/td\u003e\n        \u003ctd\u003eS\/N 0L010042 - 0L340999\u003c\/td\u003e\n      \u003c\/tr\u003e\n       \u003ctr\u003e\n        \u003ctd\u003eMerCruiser\u003c\/td\u003e\n        \u003ctd\u003e3.0L LX \/ L4 - 181\u003c\/td\u003e\n        \u003ctd\u003e140\u003c\/td\u003e\n        \u003ctd\u003e4-Stroke\u003c\/td\u003e\n        \u003ctd\u003e1990–1997\u003c\/td\u003e\n        \u003ctd\u003e59078T3\u003c\/td\u003e\n        \u003ctd\u003eS\/N 0C856559 - 0K999999\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eMerCruiser\u003c\/td\u003e\n        \u003ctd\u003e165 \/ L6 - 250\u003c\/td\u003e\n        \u003ctd\u003e165\u003c\/td\u003e\n        \u003ctd\u003e4-Stroke\u003c\/td\u003e\n        \u003ctd\u003e1970–1971\u003c\/td\u003e\n        \u003ctd\u003e59078T3\u003c\/td\u003e\n        \u003ctd\u003eS\/N 2770032 - 5813433, Also fits inline 6 engines\u003c\/td\u003e\n      \u003c\/tr\u003e\n    \u003c\/tbody\u003e\n  \u003c\/table\u003e\n\u003c\/div\u003e\n\n\u003ch2\u003eTechnical Specs\u003c\/h2\u003e\n\u003cdl\u003e\n  \u003cdt\u003eProduct Type\u003c\/dt\u003e\n\u003cdd\u003eThermostat Kit with Gasket\u003c\/dd\u003e\n  \u003cdt\u003eInstallation\u003c\/dt\u003e\n\u003cdd\u003eDirect Replacement\u003c\/dd\u003e\n  \u003cdt\u003eHousing Material\u003c\/dt\u003e\n\u003cdd\u003eMetal\u003c\/dd\u003e\n  \u003cdt\u003eOperating Temperature\u003c\/dt\u003e\n\u003cdd\u003e140°F (60°C)\u003c\/dd\u003e\n  \u003cdt\u003eOther Technical Attributes\u003c\/dt\u003e\n\u003cdd\u003eFits engines with serial numbers below 0L340999\u003c\/dd\u003e\n\u003c\/dl\u003e\n\n\u003ch2\u003eWhat’s in the Box\u003c\/h2\u003e\n\u003cul\u003e\n  \u003cli\u003e\n\u003cspan style=\"color:#d0021b;\"\u003e1 x\u003c\/span\u003e Thermostat (Replaces 59078, 18-3550)\u003c\/li\u003e\n  \u003cli\u003e\n\u003cspan style=\"color:#d0021b;\"\u003e1 x\u003c\/span\u003e Cover Gasket (Replaces 27-47590-1, 18-2831)\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eInstallation Notes\u003c\/h2\u003e\n\u003col\u003e\n    \u003cli\u003eEnsure the engine is completely cool before beginning. Disconnect the battery.\u003c\/li\u003e\n    \u003cli\u003eDrain the engine coolant into a suitable container. Locate the thermostat housing on the engine.\u003c\/li\u003e\n    \u003cli\u003ePlace a rag under the housing to catch any residual coolant. Remove the bolts securing the housing cover.\u003c\/li\u003e\n    \u003cli\u003eCarefully lift the cover and remove the old thermostat and gasket. Note the thermostat's orientation.\u003c\/li\u003e\n    \u003cli\u003eThoroughly clean both the housing and cover mating surfaces. Remove all traces of the old gasket.\u003c\/li\u003e\n    \u003cli\u003eInstall the new thermostat in the correct orientation. Place the new gasket onto the housing.\u003c\/li\u003e\n    \u003cli\u003eRe-install the housing cover and tighten the bolts to the manufacturer's specified torque.\u003c\/li\u003e\n    \u003cli\u003eRefill the cooling system with the appropriate coolant. Reconnect the battery.\u003c\/li\u003e\n    \u003cli\u003eStart the engine and check for leaks around the housing. Allow the engine to reach operating temperature, monitoring the temperature gauge.\u003c\/li\u003e\n\u003c\/ol\u003e\n\n\u003ch2\u003eTroubleshooting \u0026amp; FAQ\u003c\/h2\u003e\n\u003cdetails\u003e\n  \u003csummary\u003eHow do I know if my thermostat is failing?\u003c\/summary\u003e\n  \u003cp\u003eCommon signs of a faulty thermostat include engine overheating, the temperature gauge reading unusually low, or coolant leaks around the housing. The engine may also take a very long time to warm up.\u003c\/p\u003e\n\u003c\/details\u003e\n\u003cdetails\u003e\n  \u003csummary\u003eIs 140°F the correct temperature for my engine?\u003c\/summary\u003e\n  \u003cp\u003eYes, this 140°F (60°C) thermostat is the specified temperature rating for the compatible MerCruiser 2.5L and 3.0L engines, ensuring it does not run too cold, which is crucial for engine health and efficiency.\u003c\/p\u003e\n\u003c\/details\u003e\n\u003cdetails\u003e\n  \u003csummary\u003eMy serial number is not listed. Will this part still fit?\u003c\/summary\u003e\n  \u003cp\u003eThis kit is confirmed to fit engines with serial numbers below 0L340999. If your serial number is higher or not listed, this part may not be compatible. We recommend checking your OEM service manual or contacting us with your engine details before purchasing.\u003c\/p\u003e\n\u003c\/details\u003e\n\u003cdetails\u003e\n  \u003csummary\u003eCan I reuse my old gasket?\u003c\/summary\u003e\n  \u003cp\u003eNo. It is critical to always use a new gasket when replacing a thermostat to ensure a proper seal and prevent coolant leaks. This kit conveniently includes the correct gasket.\u003c\/p\u003eAIN\n\u003c\/details\u003e\n\u003cdetails\u003e\n  \u003csummary\u003eWhy is my engine still overheating after replacing the thermostat?\u003c\/summary\u003e\n  \u003cp\u003eIf overheating persists, other cooling system components may be at fault. Check for a worn impeller, clogged heat exchanger or risers, air pockets in the cooling system, or a faulty circulation pump.\u003c\/p\u003e\n\u003c\/details\u003e\n\u003cdetails\u003e\n  \u003csummary\u003eOEM vs. Aftermarket?\u003c\/summary\u003e\n  \u003cp\u003eThis listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.\u003c\/p\u003e\n\u003c\/details\u003e\n\u003cdetails\u003e\n  \u003csummary\u003eShipping speed \u0026amp; cost\u003c\/summary\u003e\n  \u003cp\u003eWe ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.\u003c\/p\u003e\n\u003c\/details\u003e\n\u003cdetails\u003e\n  \u003csummary\u003eWarranty \u0026amp; returns\u003c\/summary\u003e\n  \u003cp\u003eCovered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused \u0026amp; in original condition). See our policy page or reach out for instructions.\u003c\/p\u003e\n\u003c\/details\u003e\n\n\u003ch2\u003eSafety \u0026amp; Important Info\u003c\/h2\u003e\n\u003cul\u003e\n  \u003cli\u003eVerify images\/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.\u003c\/li\u003e\n  \u003cli\u003eThis is an aftermarket replacement, not an OEM component.\u003c\/li\u003e\n  \u003cli\u003eProfessional installation is recommended for safety and performance.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eAbout Sea Sierra\u003c\/h2\u003e\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003ePro-Grade Standards\u003c\/strong\u003e — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eFast Global Fulfillment\u003c\/strong\u003e — Free standard delivery to most regions in 5–15 days.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eManufacturer-Direct Value\u003c\/strong\u003e — Fair pricing without compromising on materials or QC.\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c\/section\u003e","brand":"SeaSierra","offers":[{"title":"Default Title","offer_id":50718266687803,"sku":"21TH004-G","price":20.49,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0826\/8673\/8747\/files\/21TH004-G_1_c1444464-ffb9-4be3-a7a3-8fc81dc25197.jpg?v=1762958560","url":"https:\/\/seasierra.com\/products\/thermostat-gasket-kit-fits-mercruiser-2-5l-120hp-3-0l-140hp-replaces-59078t3-59078a1-59078q3-59078b-59078","provider":"SeaSierra","version":"1.0","type":"link"}