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:

"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.

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.[1][2]

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.[1][2]

Buick/Rover V8 pattern (1961-63/1967-2004)

Cadillac V8 pattern (1955-1967)

Early Cadillacs manufactured before 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.

Oldsmobile (1961-1964)

Unique pattern, flush mount. Late 371 and 394 engines.

Pontiac V8 "Small Square" pattern (1955-1960)

Also called the "Box" pattern. Pontiac V8s were optional in 1955-1959 GMC Blue-Chip light trucks, which got larger bells to accommodate 12" truck clutches versus the 11" car clutches.

Pontiac V8 "Large Square" pattern (1961-1964)

Pontiac widened the mounting surface. Compared to the previous Small Square pattern, the mounting face is wider, and both the bolt holes and alignment pins are slightly farther apart. The changes were made to provide better stability for the newer, larger transmissions being introduced (like the Slim Jim Hydramatic).

Chevrolet V8 pattern (1955-1996)

This was so named because it was introduced on the famous Small-block engine in 1955, then spread to Chevrolet's big-block V8 and more. It became the longest lasting pattern, unaltered thru the first two generations of small-block (1996), with only minor modifications for generations III thru V (2025+).

NOTE: 1980-1983 Jeeps equipped with the Iron Duke used a custom Chrysler 904 Torqueflite automatic transmission with an integral Chevrolet bellhousing.

STANDARDIZATION

GM divisions did not share a common bell housing interface until the introduction of the BOP (Buick, Oldsmobile, Pontiac) pattern in 1964. Except for remaining pre-1964 legacy engines, standardization amongst the BOP divisions was complete by 1965, with Cadillac joining in 1968. This reduced 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 casting has distinct twin peaks containing the two top bolts, 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.

BOP V8 (Buick, Oldsmobile, Pontiac) pattern (1964-90)

The BOP divisions (Buick, Oldsmobile, Pontiac) standardized on a common pattern in 1964 (older bells continued on pre-1964 legacy engines). Cadillac joined them in 1968.

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.

The next wave of bellhousings was prompted by the conversion to smaller engines and metric dimensions, beginning with introduction o=f the GM Metric pattern in 1980.

GM metric pattern (1980-2010)

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).

GM Quad-4 pattern (1987-2002)

Northstar pattern (1993-2011)

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.

GM LS metric V8 pattern (began 1997)

For GM small-block V8s, the Chevrolet SBC comprised the first two generations (Gen I/II), and the LS family was the next evolution that spanned third, fourth, and fifth generations. Technically, the "LS pattern" isn't completely new, but rather a significant revision of the 1955 Chevy pattern with three primary changes:

  • Hole sizes were enlarged from 3/8 SAE to 10mm metric
  • To make room for the LS engine's redesigned casting, the 2 o’clock bolt hole was removed

For 1997-2013 Gen III/IV (LS1, LS2, LS3, LS7):

  • A top-center bolt hole was added at the 12 o’clock "peak" to increase block-to-transmission rigidity

In 2014, GM modified the pattern again for the 2014-present Gen V engines (LT1, L83, L86):

  • The 12 o’clock bolt hole moved about an inch to the right.

NOTE: LS crankshaft flanges sit about 0.400 inches further forward (deeper into the block) than a SBC. attachment points for cast oil pans to lower bellhousing extensions, to reduce NVH.

GM Ecotec L4 DOHC pattern (2000-2019)

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

Atlas Inline 4/5/6 pattern (2002-2012)

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

GM High Feature V6 60° DOHC pattern (2004-2025)

The "HFV6" family marked a third significant shift in GM bellhousing evolution, after the vintage Chevy pattern, and the 1980-era "GM Metric" pattern. GM engineers created a unique pattern for this engine family to handle the higher torque and NVH (Noise, Vibration, Harshness) requirements. GM Metric (Small Corporate)1980–2010GM High Feature V63.6L (LY7, LFX), 3.0L, 2.8L TurboHigh Feature (HF) Pattern2004–PresentKey Differences in the 2004+ ChangeBolt Placement: The HFV6 pattern is slightly larger and "stouter" than the older Metric pattern. While they look similar at a distance, the bolt spacing is off by just enough to prevent them from interchanging.Structural Oil Pan: One of the biggest changes in 2004 was the move toward a structural oil pan. On the High Feature V6, the bottom of the bellhousing often bolts directly into the engine's aluminum oil pan to create a rigid engine-transmission unit.Starter Mounting: The starter pocket on the High Feature V6 is specifically cast for high-torque modern starters, whereas the 1980 Metric pattern was designed for the older, bulkier permanent-magnet starters.Why was this change made?In 2004, GM was moving toward a global engine platform (the "Alloytec" in Australia/Europe). Because this engine was designed to be used in everything from front-wheel-drive Malibus to rear-wheel-drive Cadillacs and even mid-engine platforms, they moved away from the 1980-spec "Metric" pattern to a more robust design that could handle all-wheel-drive transfer cases and higher-output transmissions like the 6L80.Identifying the Patterns at a GlanceThe 1980 "Metric" Pattern: Has a very distinct "small" look. It’s almost a perfect semi-circle with two top bolts very close together.The 2004 "High Feature" Pattern: Looks more "squared off" at the top compared to the Metric. It often features a unique "ribbed" casting on the transmission side to reinforce the larger bolt spread.

  • 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

GM Ecotec Gen III L4 DOHC pattern (2013-2025)

The completely re-engineered Generation III Ecotec includes a new uniquely shaped bellhousing pattern.[3]

References

  1. ^ a b https://www.nailheadbuick.com/transmission-options-for-the-nailhe
  2. ^ a b https://www.nailheadbuick.com/transmission-adaptors-bell-housings
  3. ^ "LTG FWD | performance-engines". 2020-06-23. Archived from the original on 2020-06-23. Retrieved 2020-06-23.

[[List of GM engines|

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