Just to reaffirm what Fr. Frank is saying about the importance maintaining your cooling system - impeller, thermostats, and poppet valve . . . just look at the warranty statement from OCC on their rebuilds.
http://occcranks.com/warranty_info.html
*NOTE: The number one (1) journal on V-6 Mercury crankshafts is not included in the one year breakage warranty.
#1 is the cylinder that goes on the 2.5L motors . . . usually b/c of a stuck thermostat, worn impeller, or bad poppet valve.