TorqueList Technical Reference
Small-Block Chevrolet
Chevy 265
The small-block that started a dynasty
265
Cubic inches
Why it matters
Key takeaway
Introduced for 1955, the 265 established the compact, lightweight architecture that became the foundation of Chevrolet small-block performance for decades.
Key variants
Engineering
Technical specification
- Architecture
- 90° OHV V8 · Gen I Chevrolet small-block
- Block
- Cast iron
- Cylinder heads
- Cast iron in production applications
- Valvetrain
- Pushrod OHV · 2 valves per cylinder
- Aspiration
- Naturally aspirated
- Fuel delivery
- Carbureted; mechanical fuel injection appeared on select Corvette versions
- Firing order
- 1-8-4-3-6-5-7-2
- Main bearings
- 5
The original small-block package emphasized compact dimensions, low mass and a short 3.00-inch stroke. Early accessory and oiling details differ from later Gen I combinations.
Builder reference
Internal Specifications
Variant-specific values are labeled. For engine families with multiple factory combinations, the exact RPO/code/year takes priority over the family summary.
Identification & Output
- Engine codes / RPO
- FG · 225 hp automatic, FK · 210 hp automatic, GR · 225 hp manual, GU · 240 hp manual, GV · 210 hp manual
- Casting numbers
- 3720991 (1956 Corvette block reference), 3725306 / 3731762 (1956 Corvette head references)
- Power rating
- SAE gross (period factory ratings)
- Horsepower RPM
- 210 hp @ 5200 rpm single-four; 225 hp @ 5200 rpm dual-quad; 240 hp @ 5600 rpm high-lift-cam 1956 Corvette versions
- Torque RPM
- 270 lb-ft @ 3200–3600 rpm on 210/225-hp 1956 Corvette versions
Rotating Assembly
- Crankshaft
- Drop-forged steel crankshaft in 1956 Corvette applications
- Connecting rods
- Drop-forged steel connecting rods · 5.70-in center-to-center factory dimension
- Rod length
- 5.70 in
- Pistons
- Cast-alloy aluminum slipper-skirt pistons with steel struts in 1956 factory specification
- Piston pin
- 0.927 in nominal small-block pin diameter
- Balance
- Internally balanced
- Rear main seal
- 2-piece rear main seal
- Deck height
- 9.025 in nominal Gen I small-block deck height
- Main journal
- 2.30 in
- Rod journal
- 2.00 in
Cylinder Heads
- Chamber
- 56 cc on the documented 3725306 1956 Corvette head; verify chamber on other 265 castings
- Intake valve
- 1.72 in on 1956 Corvette production heads
- Exhaust valve
- 1.50 in on 1956 Corvette production heads
Cam & Valvetrain
- Camshaft
- Mechanical-lifter performance cam in 1956 Corvette versions; GU high-output version used the special high-lift Duntov cam
- Cam lift
- 0.4043 in intake/exhaust on documented 1956 Corvette performance cams
- Rocker ratio
- 1.50:1 stamped-steel rocker architecture
Operating Specs
- Oil capacity
- 4 qt crankcase refill, less filter (1956 Corvette reference)
- Induction
- Carter WCFB single four-barrel or dual four-barrels depending on 1956 Corvette version
- Ignition
- Delco points-style distributor
- Production notes
- 1956 Corvette production totaled 3,467 cars. RPO 469 (265/225 hp) was installed in 3,080 cars, while the special high-lift camshaft option RPO 449 was installed in 111. Engine suffixes FG/FK/GR/GU/GV identify the exact 210-, 225- and 240-hp/transmission combinations.
Service Specs
- Spark plug
- AC 43-5R (1956 Corvette reference)
- Spark-plug gap
- 0.033–0.038 in factory range
- Ignition timing
- 4° BTDC at idle (1956 Corvette reference)
- Oil pressure
- 30 psi @ 1170–1200 rpm (1956 Corvette reference)
- Coolant capacity
- 16 qt without heater / 17 qt with heater
- Thermostat
- 160°F opening temperature
- Fuel pressure
- 4.0–5.25 psi mechanical fuel-pump pressure (1956 Corvette reference)
Compatibility Notes
- Early 265 small-block hardware changed quickly across 1955–1957; verify the exact model year before mixing intake, ignition, accessory-drive, flywheel/flexplate, or mounting hardware.
- 1955 production is especially distinct from later Gen I small-block service parts, so restoration and swap builds should be matched to the block's year and casting rather than displacement alone.
Factory fitment
Notable applications
Versions that matter
Key factory / build variants
1955 passenger-car 265
The first production Chevrolet small-block V8.
1955
Multiple 2-bbl and 4-bbl ratings
Corvette performance 265
Helped move Corvette toward serious V8 performance.
1955–1956
Higher-output carbureted combinations
Late 265 development
Final evolution before the 283 became the headline small-block.
1957
Application-dependent
Strengths
- Compact and lightweight for its era
- Simple architecture
- Huge historical importance
Watch-outs
- Small displacement limits torque compared with later small-blocks
- Early oiling and accessory details differ from later SBC combinations
Common upgrades
- Period-style four-barrel intake
- Cylinder-head and valvetrain upgrades
- Exhaust improvements
Builder notes
Engineering notes worth knowing
Early 265 blocks did not use an integral oil-filter boss like later small-blocks.
The 3.75-inch bore is smaller than later 4.00-inch-bore performance SBCs, which affects cylinder-head and valve choices.
Restoration builds need attention to year-specific front-drive, oiling and mounting details.
History & context
The story behind the engine
The 265 was more than Chevrolet's new V8 for 1955. It established the compact architecture that let Chevrolet scale one basic engine family from economy cars to Corvettes, drag cars and decades of hot-rodding.
Racing history
The early small-block architecture quickly proved adaptable to racing and laid the groundwork for Chevrolet's later high-revving small-block programs.
Legacy
The 265 is where the classic Chevrolet small-block story begins.
Documented motorsport results
Famous Race Wins
1956
1st in C Production · 2nd overallPebble Beach National Championship Sports Car Road Races
Driver
Dr. Dick Thompson
Chevrolet Corvette C1 · 265 V8
Thompson led much of the race and finished first in C Production at Pebble Beach, helping prove Chevrolet’s new 265-powered Corvette could compete seriously on a road course.
Verification
Reference sources
GM Heritage Archive vehicle information kits
manufacturer archive
Wikimedia Commons GM Heritage Center 265 engine image
licensed media
1956 Chevrolet factory engine specifications
manufacturer specification
1956 Corvette specifications and service data
historical specification
Corvette Action Center — 1956 Corvette production and engine suffixes
historical specification
Keep reading
Related stories
Origin story
01How the 265 rewrote Chevrolet performance
Why Chevrolet's compact 1955 V8 changed the company's performance direction and became the blueprint for generations of small-blocks.
Engineering
02How the small-block grew without losing its footprint
The bore-and-stroke changes that carried the family from 265 cubic inches into the 283, 327, 350 and beyond.
Corvette history
03The V8 that transformed the early Corvette
How the 265 helped turn Chevrolet's fiberglass sports car from an interesting idea into a genuine performance platform.
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