Mercruiser Thermostat Housing - 18-3678 Mercruiser Thermostat Housing - 18-3678 by Sierra. Please use the Red PARTS CHARTS button (top of any page) to locate the correct part for your application. $ 143.29               More Information      Quantity:     Thermostat Housing - 18-3678 Thermostat Housing - 18-3678. Please use the Red PARTS CHARTS button (top of any page) to locate the correct part for your application. All prices are per unit (50' Coil=50 Units, 2 PK=2 Units, and etc.). Verify your quantity is correct at Check-Outn adjust as necessary. Send us an email (Contact Us button at top of any page) if you have any questions. $ 143.29               More Information      Quantity: