List of GM bellhousing patterns

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

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.

NOTES:

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

Chevrolet V8 pattern

This was so named because it began with Chevrolet's V8 engines.

• Chevrolet big-block V8s
• Chevrolet small-block V8s
• GM Vortec 4300 90° V6
• 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).
• Chevrolet Turbo-Thrift engine (post-1962)
• Chevrolet 153 Inline 4 (Chevy II, pre-Iron-Duke - includes the Vortec 3000/181 industrial/marine crate motor)
• Detroit Diesel V8 6.2L and 6.5L
• Duramax V8
• 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:

Inline 4

• AMC 2.5L I4 (used in Jeeps and Dodge Dakota)
• GM Iron Duke 2.5L I4 (1980 & newer metric)

V6

• Buick V6 (1988 & newer metric 3300/3800)
• GM 122 V6
• GM 60° V6 (also used by AMC)
• GM High Value OHV V6
• Isuzu 75° V6

V8

• Cadillac High Technology V8
• GM LS4 FWD V8

Northstar pattern

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.

• All Cadillac Northstar V8s
• Oldsmobile Aurora L47 V8
• GM 3.5L LX5 "Short Star" V6

Atlas pattern

Atlas family engines use a unique bellhousing pattern which no other GM motors share.

• GM Atlas engine

Buick Nailhead, Generation 1 pattern

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.

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.

Buick / Rover V8 pattern

• Buick/Oldsmobile/Pontiac 215 Aluminum V-8 (1961-1963)
• Buick 198 V6 (1962-1963)
• Rover V-8 based on the GM 215 (1967-2004)

Buick, Oldsmobile, Pontiac (BOP) V8 pattern

BOP V8 Pattern

• Buick V6s (225 / 231 / 3.8) (1964-1988)
• Post-1963 Buick Small Block V8s (1964-1982)
• Buick Big Block V8s (1967-1976)

• Jeep "Dauntless" Buick engines: the 225  V6 (1967-1974) and 350 V8 (1969-1970)
• Cadillac cast iron V8s after 1967 (1968-85 472 and 500, 368 and 425)
• Oldsmobile Gen 2 V8s (1964-1990)
• Pontiac V8 engines (Some 1964 389s, all 1965-1981)

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.

• Oldsmobile V8s (1949-60) 303, 324, early 371
• Cadillac V8s (1949-54) early 331

Oldsmobile V8 1961-1964

Unique pattern, flush mount. Late Gen1 V8s: 371, 391

Cadillac V8 pattern, 1955-1967

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.

GM 4-cylinder pattern

• GM Quad-4 family

GM Ecotec 4-cylinder pattern

An example of this pattern can be seen to the right.

• GM Ecotec family (Generation I and II)
• GM LLT and late High Feature family

GM Ecotec Generation III pattern

The completely re-engineered Generation III Ecotec inline 4 includes a new uniquely shaped bellhousing pattern.

GM High Feature V6 pattern

• Cadilac 2.8
• Holden 3.2
• 3.0 LF1,LFW and 4TH generation LGW,LGY
• 3.6 LY7,LLT,LFX,LWR,LCS,LF3,LF4,LFR,LFY and 4TH generation LGX,LGZ

GM High Feature engine

References

[[List of GM engines|