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/151 in³ (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. Rear wheel drive applications have the starter mounted on the right side of the block (when viewed from the flywheel) and on the opposite side of the block compared to front wheel drive installations. The 2.2l S10/Sonoma had the starter located in the same position as front wheel drive cars. A rear wheel drive bellhousing is displayed at right, and the integrated front wheel drive bellhousing is displayed at the lower right (in this case, as a part of the GM 6T70 Transmission).
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.[3][4]
Starters are on the left (driver's) side on Olds 307-455 and Pontiac and the right (passenger) side on Cadillac 425/472/500 and Buick 225/231/3800/300/340/350/400/430/455.
Four 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.
Cadillac V8 pattern, pre-1967
Early Cadillacs manufactured before 1965 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.
