GMC · GMC Raised-Deck Inline-Six
GMC 302 Inline-Six
The 4.00 x 4.00-inch GMC six built for military and civilian trucks and prized by inline-six hot rodders.
301.6
Cubic inches
Originally engineered for military 2½-ton trucks, the square-bore/stroke 302 became the largest classic GMC straight-six and one of the strongest American inline-six hot-rod foundations of its era.
Displacement
301.6 ci
Bore
4"
Stroke
4"
Years
1952–1960 core production; civilian 302 production 1952–1959
Key Takeaway
The 302 combined a 4.00-inch bore with a 4.00-inch stroke, giving GMC’s largest classic straight-six square dimensions. Built on the raised-deck family, it used heavy-duty cast-iron construction and was engineered for military and commercial service before becoming a favorite performance foundation.
Technical Specifications
- Horsepower
- 145 hp in 1953 truck listing; about 160 hp in 1959 civilian specification
- Torque
- About 268 lb-ft in 1959 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 302 combined a 4.00-inch bore with a 4.00-inch stroke, giving GMC’s largest classic straight-six square dimensions. Built on the raised-deck family, it used heavy-duty cast-iron construction and was engineered for military and commercial service before becoming a favorite performance foundation.
Internal Specifications
Variant-specific values are labeled. Engine families that span multiple years or RPOs can have different heads, cams, induction and rotating parts, so the exact code/year always takes priority.
Operating Specs
- Production notes
- The 302 was developed for military M135/M211 duty and then sold in civilian GMC trucks during the 1950s. Civilian production ended after 1959 as GMC transitioned to the new 60-degree V6 family.
Notable Applications
Strengths
- 4.00 x 4.00-inch square dimensions
- Heavy-duty block
- Excellent vintage inline-six performance reputation
Weaknesses / Watch-outs
- Heavy and physically long
- Original cores and performance parts increasingly scarce
- Not interchangeable with later Chevrolet 292 family
Common Upgrades
- Multi-carb intake
- Ported cylinder head
- Headers
- Performance camshaft
Racing History & Legacy
The 302 became the best-known engine in the classic GMC straight-six family. Beyond its military and commercial service, its reputation for strength and breathing potential gave it a second life as a vintage performance engine.
References & Image Credit
Engine image: U.S. Army · Public domain · source
Engineering Notes Worth Knowing
4.00 x 4.00-inch bore and stroke; 301.6 cu in / 4.9 L.
Raised-deck GMC OHV inline-six with square bore/stroke geometry.
Period output ranged from about 145 hp in early-1950s truck listings to roughly 160 hp in later civilian specifications.
Applications included M135/M211 military trucks and GMC 450-series commercial trucks.
History · context · related reading
The Story Behind GMC 302 Inline-Six
GMC’s 302 started life with serious work in mind, including military 2½-ton truck duty. Its large displacement, square dimensions, and robust construction later made it one of the most respected American inline-six engines among traditional hot rodders.
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