Catalogs
TorqueList Garage Reference Series

Volume 1 · GMC

Engine
Reference Catalog

Factory specifications, architecture, key variants, engineering notes, racing history and the stories that made GMC's most important engines matter.

Technical reference•Historical context•Builder notes•Print ready

Edition 1.0 · September 2026 · Published

Public preview · first 3 engine profiles unlocked

13

Detailed engine profiles

2/13

Commercially cleared photos

32

Source references

V1

7 engine families

Publication notice

Independent TorqueList reference

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TorqueList is an independent automotive reference publication and is not affiliated with, sponsored by, endorsed by, or authorized by GMC, General Motors, or any other vehicle or engine manufacturer.

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© 2026 TorqueList. Original editorial text, compilation, selection, organization, layout, and TorqueList-created graphics are protected by copyright. Underlying facts, specifications, and manufacturer-identifying terms remain the property or factual subject matter of their respective owners.

Accuracy

Specifications can vary by model year, application, option code, market, and production revision. Verify critical service, purchasing, racing, and installation information against the exact factory documentation for the specific vehicle or engine before relying on it.

Media policy

Third-party photographs appear only when their commercial-use basis is recorded. Creator, source, and license information is retained with each approved image. Engines without an approved third-party photograph use TorqueList-owned original technical artwork.

Contents

GMC Engine Reference — Volume 1

Profiles are organized by engine family and pull from the same published TorqueList Engine Library used by the web experience.

GMC Low-Deck Inline-Six

2 profiles
GMC 228 Inline-Six1939–1953
228 ci
GMC 248 Inline-Six1939–1955
248 ci

GMC Raised-Deck Inline-Six

2 profiles
GMC 270 Inline-Six1941–1963
270 ci
GMC 302 Inline-Six1952–1960 core production; civilian 302 production 1952–1959
301.6 ci

GMC 60-Degree V6

4 profiles
GMC 305 V61960–1974 family/application era
305 ci
GMC 351 V61960–1972 core gasoline era
351.2 ci
GMC 401 V61960–1975 family era
400.9 ci
GMC 478 V61962–1973 core gasoline production
477.7 ci

GMC Magnum V6

1 profiles
GMC 432 V61973–1974
432.3 ci

GMC Heavy-Duty V8

1 profiles
GMC 637 V81966–1972
637 ci

GMC Twin-Six

1 profiles
GMC 702 Twin-Six V121960–1965
702.4 ci

Toro-Flow Diesel

2 profiles
GMC D351 Toro-Flow Diesel V61960s–early 1970s Toro-Flow era
351.2 ci
GMC D478 / DH478 Toro-Flow Diesel V61960s–early 1970s Toro-Flow era
477.7 ci
13/13 profiles include reference sources.
Every engine has artwork; rights-cleared real photos replace TorqueList original technical art as they are approved.
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01

TorqueList Technical Reference

GMC Low-Deck Inline-Six

Full web profile

GMC 228 Inline-Six

GMC's own light-truck six that replaced the earlier Pontiac-sourced engine and anchored prewar/postwar pickups.

TorqueList Original Technical Art

GMC 228 Inline-Six

Inline6 · N/A

3.7L
★

Why it matters

Key takeaway

The 228 marked GMC's move to an in-house light-truck six and established the low-deck architecture shared with the 248.

Engineering

Technical specification

Architecture
OHV inline six, low-deck GMC family
Block
Cast iron
Cylinder heads
Cast iron
Valvetrain
Pushrod OHV
Aspiration
Naturally aspirated
Fuel delivery
Carbureted

The 228 used GMC’s early low-deck OHV straight-six layout with a 3.5625-inch bore and 3.8125-inch stroke. Its undersquare geometry, cast-iron construction, pushrod valvetrain, and carbureted induction were aimed at dependable low-speed truck service rather than high-rpm power.

Factory fitment

Notable applications

GMC light-duty pickupsGMC 100–250 series trucksPrewar and postwar GMC commercial vehicles

Strengths

  • Simple durable truck architecture
  • Good low-speed torque
  • Strong period GMC identity

Watch-outs

  • Four-main-bearing low-deck design
  • Limited modern performance aftermarket
  • Lower displacement than later GMC sixes

Common upgrades

  • Electronic ignition conversion
  • Carburetor restoration
  • Dual exhaust or period intake upgrades

Builder notes

Engineering notes worth knowing

01

3.5625 x 3.8125-inch bore and stroke; 228 cu in / 3.7 L.

02

Low-deck GMC OHV inline-six architecture with cast-iron block and head.

03

Period output was roughly 95–100 hp depending on year, with about 182 lb-ft commonly cited for later specifications.

04

Used across GMC light-duty pickups and 100–250 series commercial trucks.

History & context

The story behind the engine

Introduced before World War II and carried well into the postwar truck boom, the 228 helped establish GMC’s own six-cylinder identity. It was sized for light and commercial truck work, with simple construction and torque-focused manners that fit the job GMC expected it to do.

Legacy

The 228 became the starting point for GMC’s low-deck inline-six family and directly set the pattern for the larger 248. Its importance is less about headline horsepower than about establishing the durable GMC truck-six formula that followed.

Verification

Reference sources

GMC Truck Club — 1949–55 engine history

club history

Old GMC Trucks — casting/year reference

technical reference

TorqueList independent reference · Not an OEM publication or endorsement. Original TorqueList editorial and layout rights reserved.

02

TorqueList Technical Reference

GMC Low-Deck Inline-Six

Full web profile

GMC 248 Inline-Six

The larger low-deck GMC six used in heavier light- and medium-duty trucks before the 270 took over.

TorqueList Original Technical Art

GMC 248 Inline-Six

Inline6 · N/A

4.1L
★

Why it matters

Key takeaway

The 248 enlarged the 228's low-deck layout and became a common medium-duty GMC six through the first half of the 1950s.

Engineering

Technical specification

Architecture
OHV inline six, low-deck GMC family
Block
Cast iron
Cylinder heads
Cast iron
Valvetrain
Pushrod OHV
Aspiration
Naturally aspirated
Fuel delivery
Carbureted

The 248 enlarged the low-deck GMC straight-six while keeping the same 3.8125-inch stroke used by the 228. A larger 3.71875-inch bore raised displacement and torque without changing the basic cast-iron, pushrod-OHV, carbureted truck architecture.

Factory fitment

Notable applications

GMC 300–400 series trucksGMC medium-duty trucksGMC pickups in selected applications

Strengths

  • More displacement than 228
  • Durable truck-focused design
  • Good low-speed torque

Watch-outs

  • Four-main-bearing architecture
  • Low-deck family limits later parts interchange
  • Modest top-end breathing

Common upgrades

  • Electronic ignition
  • Carburetor tuning
  • Period dual-carb or exhaust upgrades

Builder notes

Engineering notes worth knowing

01

3.71875 x 3.8125-inch bore and stroke; 248 cu in / 4.1 L.

02

Low-deck GMC OHV inline-six with cast-iron block and cylinder head.

03

Factory listings moved from about 100 hp in 1949 to 114 hp by 1953.

04

Commonly found in GMC 300–400 series and other medium-duty truck applications.

History & context

The story behind the engine

The 248 was GMC’s straightforward answer to customers who needed more from the basic low-deck six without moving to a completely different engine family. It kept the familiar architecture of the 228 but added displacement and useful truck torque for heavier work.

Legacy

The 248 represents the mature form of GMC’s low-deck six before the raised-deck 270 and 302 took over the higher-capacity role. It remains an important link in the company’s transition from prewar truck engines to the heavier postwar GMC sixes.

Verification

Reference sources

GMC Truck Club — 1949–55 engine history

club history

1956 GMC Models and Features brochure — 248 replacement and prior output reference

manufacturer brochure

TorqueList independent reference · Not an OEM publication or endorsement. Original TorqueList editorial and layout rights reserved.

03

TorqueList Technical Reference

GMC Raised-Deck Inline-Six

Full web profile

GMC 270 Inline-Six

The long-lived raised-deck GMC six that became a favorite in trucks and early inline-six hot-rodding.

TorqueList Original Technical Art

GMC 270 Inline-Six

Inline6 · N/A

4.4L
★

Why it matters

Key takeaway

The 270 moved GMC to the taller raised-deck six and became the bridge between conventional truck duty and the more performance-oriented 302.

Engineering

Technical specification

Architecture
OHV inline six, raised-deck GMC family
Block
Cast iron
Cylinder heads
Cast iron
Valvetrain
Pushrod OHV
Aspiration
Naturally aspirated
Fuel delivery
Carbureted

The 270 moved GMC into the taller raised-deck straight-six family and used a full 4.00-inch stroke with a 3.78125-inch bore. That long-stroke geometry favored torque and commercial durability, while the taller block created room for the larger 302 that followed.

Factory fitment

Notable applications

GMC 400/450 series trucksGMC pickups and medium-duty trucksGMC Value Van in later applications

Strengths

  • Strong 4.00-inch stroke torque
  • Heavy-duty construction
  • Good vintage hot-rod reputation

Watch-outs

  • Large and heavy
  • Lower breathing potential than 302
  • Long-stroke design favors torque over rpm

Common upgrades

  • 302 cylinder-head/intake strategies where compatible
  • Dual carburetors
  • Headers and ignition upgrades

Builder notes

Engineering notes worth knowing

01

3.78125 x 4.00-inch bore and stroke; 270 cu in / 4.4 L.

02

Raised-deck GMC OHV inline-six architecture.

03

Period ratings ranged from roughly 104 hp in 1949 to about 133 hp in later specifications.

04

Used in GMC pickups, 400/450 series trucks, medium-duty trucks, and later van applications.

History & context

The story behind the engine

The 270 was where GMC’s straight-six program grew into a more serious heavy-duty design. Its taller deck and four-inch stroke gave it the low-rpm character truck buyers needed, while also creating the architecture that performance-minded builders would later associate with the 302.

Legacy

The 270 is remembered as the bridge between GMC’s early low-deck sixes and the legendary 302. It carried the raised-deck family into mainstream truck use and became a respected vintage hot-rod foundation in its own right.

Verification

Reference sources

GMC Truck Club — engine history

club history

Old GMC Trucks — 270 casting history

technical reference

TorqueList independent reference · Not an OEM publication or endorsement. Original TorqueList editorial and layout rights reserved.

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All-manufacturer workshop tools

GMC Workshop Companion

Builder utilities and edition 2.0 reference sections. Calculations use your entered measurements; empty or unverified interchange, machining, power and torque data are never represented as confirmed specifications.

Public preview: workshop reference content remains limited until this volume is unlocked. The compression calculator is a general educational utility.

Engine geometry & static compression

Starting values are examples only, not specifications for any engine. Use measured dimensions. Dish and valve relief add clearance volume; dome subtracts it. Deck means piston below deck at top dead center. Calculations are for one cylinder except total displacement.

Geometry
Compression inputs

Static compression

9.63:1

Displacement

349.8 ci

Displacement

5.733 L

Mean piston speed

3,480 ft/min

Rod-to-stroke ratio

1.638

Clearance volume

83.02 cc / cyl

Static compression = (swept + clearance) ÷ clearance. Mean piston speed = 2 × stroke (feet) × RPM. These calculations do not predict fuel octane, dynamic compression, safe RPM, engine strength, horsepower, or whether a combination will clear mechanically.

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Attribution record

Image & License Credits

Only photographs recorded as approved for commercial use appear in this edition. TorqueList does not claim ownership of third-party photographs; all fallback technical artwork is TorqueList-created original material.

No third-party photographs are included in this edition.

TorqueList is an independent automotive reference publication and is not affiliated with, sponsored by, endorsed by, or authorized by GMC, General Motors, or any other vehicle or engine manufacturer.