43691 Mercruiser BOLT, (.500-13 x 4.00)

Water Temperature Regulators (Thermostats)
Replace
Replacing your thermostats prior to failure is a recommended preventive maintenance practice because it reduces the chances for unscheduled downtime.Depending on load, a thermostat that fails in a partially opened position will cause either an overheating or an over cooling condition.If the thermostat fails in the closed position, it will cause excessive overheating. Excessive overheating could result in cylinder head cracking or piston seizure problems.If the thermostat fails in the open position, it will cause the engine operating temperature to be too low during partial load operation. Low engine operating temperatures during partial loads could cause an excessive carbon build-up inside the cylinder. This excessive carbon build-up could result in accelerated ring and liner wear.
Failure to replace your thermostat on a regularly scheduled basis could cause severe engine damage.Caterpillar engines incorporate a shunt design cooling system and require operating the engine with a thermostat installed.If the thermostat is installed wrong, the engine may overheat, causing cylinder head damage. Ensure that the new thermostat is installed in the original position. Ensure that the thermostat vent hole is open.DO NOT use liquid gasket material on the gasket or cylinder head surface.
Refer to the Service Manual for the water temperature regulator replacement procedure, or consult with your Caterpillar dealer. If replacing thermostats ONLY, drain the coolant from the cooling system to a level below the thermostat housing.Refer to the Cooling System Specifications in this publication