421 was enlarged to by increasing its bore to . Both Chevy and Ford had 427 cu in performance engines, so Pontiac simply referred to its 427 as a 428 to one-up them. It retained the 421's stroke and main journal. Offered from 1967 to 1969, it produced in 1967, in 1968 and in 1969. The crankshaft in the 428 had a "N" cast on them (designating nodular steel) as opposed to the 421's Armasteel. In 1969, Pontiac also used a revised crankshaft out of a Pearlitic malleable-iron, although it still used the "N" casting letter. This new material had stronger alloys in the iron. All 428 cylinder heads received the 14 degree valve angle, closed chamber only in 67 and open chamber 68 and up. The 428 was factory installed in large cars only. However, a few dealers would offer them in the GTO or Firebird and do their own installations, such as Royal Pontiac who offered the "Bobcat" conversion. It was replaced by the 455 for the 1970 model year. 455 The 428 was increased to in 1970. Again, its bore was expanded, this time to . The stroke increased to . It was rated at , but the Grand Prix with the same specifications was rated at . The horsepower ratings of this era were often dubious, with engines rated higher or lower in output for advertising, political, or insurance purposes. Though listed as slightly less powerful than some high-performance iterations of the 400 (such as the Ram-Air), the 455 had of torque, . The engine was available in all full-size Pontiacs. An HO version could be ordered in the GTO, as GM had lifted its restrictions on offering engines larger than in mid-sized cars (resulting in the Chevrolet Chevelle, Buick Gran Sport, and Oldsmobile 442). For 1971, Pontiac introduced another High Output (HO) version with standard internal parts, a reinforced block with four-bolt main bearing caps, and improved cylinder head design with intake ports and special round exhaust ports for better breathing, yet still making just gross (or in the more accurate SAE Net system). Standard in the Firebird Trans Am, it was still a rare engine. In 1973 a further refined and even stronger but less powerful version, the Super Duty (SD) engine, was planned for Firebird, Grand Am, Le Mans (GTO), and Grand Prix models. The SD-455 used round-port cylinder heads similar to those