1961 model year. This group of cars was commonly referred to as the B-O-P group — for Buick-Olds-Pontiac — or the Y-bodies. Known variously as the Fireball and Skylark by Buick (and as Rockette, Cutlass, and Turbo-Rocket by Oldsmobile), the 215 had a bore spacing, a bore and a stroke of , for an actual displacement of . At the time, the engine was the lightest mass-production V8 in the world, with a dry weight of only . Measuring long, wide, and high (same as the small-block Chevy), it became standard equipment in the 1961 Buick Special. Oldsmobile and Pontiac each used an all-aluminum 215 on its senior compact cars, the Oldsmobile F-85, Cutlass, and Jetfire, and Pontiac Tempest and LeMans. Pontiac used the Buick version of the 215. At that time the engine was closely associated with the Buick brand, and Pontiac sold few cars with it, using it only in 1961 and 1962. The Oldsmobile version of this engine, although sharing the same basic architecture, had cylinder heads and angled valve covers designed by Oldsmobile engineers to look like a traditional Olds V8 and was produced on a separate assembly line. Among the differences between the Oldsmobile and Buick versions was weight, being somewhat heavier, at . The major design differences were in the cylinder heads. Buick used a five-bolt pattern around each cylinder, while Oldsmobile used a six-bolt pattern and a wedge combustion chamber, which allowed larger valves. The sixth bolt was added to the intake manifold side of the head, one extra bolt for each cylinder, intended to alleviate a head-warping problem on high-compression versions. This meant that Buick heads would fit on Oldsmobile blocks, but not vice versa. Changing the compression ratio on an Oldsmobile 215 required changing the heads, but on a Buick 215, only the pistons were changed, which was less expensive and simpler. For that reason, the more common Buick version (which looks like a traditional Nailhead V8) has emerged as more desirable to some. The Oldsmobile wedge-shaped/quench combustion chambers/pistons are more compatible with modern low-octane/low-lead motor fuels than the Buick 'hemispherical'-shaped combustion chambers and domed pistons. The previous statement is incorrect, the 215 Buick only used "dished head" pistons even in the highest compression models, all Buick 215's have a 37-cc wedge combustion chamber. Later Rover versions of the aluminum block and subsequent Buick iron small-block 300s with aluminum, then iron