List of GM bellhousing patterns

The following is a list of General Motors' Bellhousing patterns, i.e. the unique mounting bolt hole pattern that matches specific engines.

NOTES:

Most Automatic transmissions and a few Manual transmissions have integral bellhousings.
Starters were typically mounted to the bellhousing until moving to the engine block beginning 1964.
Some early "Extended" engine blocks were cast with integral "upper-half" bells, but most used a flat mounting surface.

"V8 INDEPENDENCE" ERA (1949-1963)

The mid-century early V8 era was a time of fierce independence between GM divisions. Back then, each GM division acted like a separate company, with their own foundries, their own engineers, and their own pride. So, the 1949 to 1964 bellhousing patterns were very much unique designs.

Early Rocket/Cad "Extended Block" V8 patterns (1949-60)

The upper half of the bell was cast into the block. Olds & Cad patterns were similar.

Oldsmobile Gen1 V8s (1949–60) 303, 324, early 371
Cadillac 331 V8 (1949–54)

Buick Nailhead, Generation 1 pattern (1953-56)

Gen1 Nailheads had a unique round-shaped bellhousing that looks almost the same as bells for the later Gen2 Nailhead, but in fact the circular flange is about 1" larger.

Buick Nailhead, Generation 2 pattern (1957-66)

Gen2 Nailheads had a unique round-shaped bellhousing that looks almost the same as bells for the earlier Gen1 Nailhead, but in fact the circular flange is about 1" smaller.

References

[[List of GM engines|