The 390 eight-cylinder engine was widely used by Ford and by the Mercury division of Ford during the 1968 production year. Although the popularity of the engine diminished with the release of the 428 engine in the middle of the year, the 390 was still a popular choice in the truck line due to the engine's high torque rating. Although the engine was used for many years, not all 390 specifications are the same.
The 390's piston stroke was 3.780 inches and its bore was 4.050 inches. The 390 was offered with either a two- or four-barrel carburetor in 1968. With the two-barrel carburetor the 390 produced a maximum of 270 horsepower and 403 ft.-lb. of torque with a 9.5:1 compression ratio. With the four-barrel carburetor the engine produced a maximum of 315 horsepower and 427 ft.-lb. of torque with a compression ratio of 10.5:1. Normal oil pressure was within a range of 35 and 55 lbs. of pressure.
The 390's piston clearance was within a range of .0015 to .0023 inch. The compression ring's end gap was .010 inch, and the oil ring's end gap was .015 inch. Both gaps were measured with the rings positioned within tapered bores and at the tightest portion of ring travel. The diameter of the wrist pin was .975 inch.
The diameter of the rod shaft was within a range of 2.4380 to 2.4388 inches. The clearance of each rod bearing was within a range of .0008 to .0015 inch.
The shaft diameter of the crankshaft bearings was within a range of 2.7484 to 2.7492 inches. The clearance of each crankshaft bearing was within a range of .0005 to .0015 inch.
The intake and exhaust valve angles were 45 degrees at the seat and 44 degrees at the face. The intake and exhaust valve lash was identical at within .100 to .200 inch when measured at the tip of the valve stem with the lifter fully collapsed. The height of the valve springs was 1.875 inches. Each spring produced 90 lbs. of pressure.
Without a heater the 390's cooling system stored 19.5 qts. of coolant. If the vehicle was equipped with a heater or air conditioning, the system held 20.5 qts. The radiator cap's relief pressure was within a range of 12 to 15 lbs. regardless of whether the engine was equipped with air conditioning. The thermostat opened at 195 degrees.
