Although the original (1949-54) Oldsmobile and Cadillac V8s used a similar extended block design, GM divisions did not share a common bell housing interface until the introduction of the BOP pattern in 1964. Within a year, the BOP divisions (Buick, Oldsmobile, Pontiac) standardized on the BOP pattern (except for on pre-1964 legacy engines). Cadillac joined them in 1968, reducing GM bellhousing patterns to just two (BOP & Chevrolet) until the GM Metric pattern was introduced in 1980.
Both the Chevy and BOP patterns are vertically symmetrical and share locations of their 2 bottom bolts and 2 locating pins; they differ in the locations of the 4 bolts above the pins. The BOP has a distinct "dip" or valley between the two top bolts (twin peaks), whereas the Chevy pattern has a single peak in the center. A few transmissions (TH200, TH350, & TH700R4), were produced with a hybrid "Uni-case"; these have a double set of bolt holes on the top, allowing it to be bolted both to Chevy and BOP engines.
GM Iron Duke RWD inline 4 (early RWD Variants, later versions may use a FWD pattern, and have two possible starter locations)
Jeep with GM Iron Duke inline 4 2.5L/151in3 (1980-1983). These use a Chrysler custom Torqueflite 904 automatic transmission with an integral Chevrolet bellhousing. Do not confuse with later AMC 2.5 L engine that uses GM small corporate pattern (see below).
Generation III V8s with modifications. These modifications include an additional bolt hole at the top of the pattern, and attachment points for cast oil pans to lower bellhousing extensions, to reduce NVH.
GM metric pattern
Also called the GM small corporate pattern and the S10 pattern
This pattern has a distinctive odd-sided hexagonal shape. As viewed from the flywheel, the starter mounted on the left side of the block for the RWD 2.2L S10/Sonoma and FWD applications, and on the right side for all other RWD applications. A rear wheel drive bellhousing is displayed at right. Applications include:
Nearly identical to the GM small corporate/metric pattern, except that the starter is located between the cylinder banks, and the lower right bolt hole is moved outward by roughly one inch. Being nearly identical, it too has the distinctive odd-sided hexagonal shape. These engines can be fitted in rear wheel drive vehicles with the right bellhousing and are used in hot rods, kit cars, sand rails and late model engine swaps.
1953-56 Nailheads have a unique round-shaped bellhousing that looks almost the same as bells for the later 1957-66 Nailhead, but in fact the circular flange is about 1" larger.[1][2]
Buick Nailhead, Generation 2 pattern
1957-66 Nailheads have a unique round-shaped bellhousing that looks almost the same as bells for the earlier 1953-56 Nailhead, but in fact the circular flange is about 1" smaller.[1][2]
Starters are on the left (driver's) side on Olds 307-455 and Pontiac, and on the right (passenger) side on Cadillac 425/472/500 and Buick 225/231/3800/300/340/350/400/430/455.
Four lower bolt holes and two locator pins are common to the Chevrolet and B-O-P patterns. Some transmissions, most notably the TH200-4R, take advantage of this by integrating both specifications into a "universal" bolt pattern casting.
"Extended" blocks pattern
Upper half of the bell was cast into the block. Olds & Cad patterns were similar.
Early Cadillac V8s manufactured fron 1955 to 1967 used a "round top" bellhousing very similar to early Buicks; around 1965, the bellhousing pattern was revised until the BOP bolt pattern was adopted in 1968.
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