GMC LG4 specs: 305 ci (5.0L) 8-cylinder, 3.736 x 3.48 in (94.9 x 88.4 mm) bore and stroke. Built 1979–1987. Used in the Buick Regal and Oldsmobile Cutlass.
The GMC Small Block Chevy (LG4) is oversquare — the 3.736" bore exceeds the 3.4799" stroke by 7.4% — a layout that favors airflow and RPM over leverage at the crank. The 3.4799" stroke puts mean piston speed at 4,000 ft/min around 6,900 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.638 is in the range most production engines fall in. Each of the 8 cylinders displaces 38.1 ci (625 cc).
It is one of 27 engines in the Small Block Chevy family, sitting between the 265 ci and 400 ci members. 7 factory variants share these dimensions across 1979–1987.
Dimensions
Displacement
305 ci / 5 L
Bore
3.736"
Stroke
3.4799"
Cylinders
8
Configuration
V8
Compression ratio
8.6–9.5:1 (published figures vary)
Rod length
5.7"
Rod ratio
1.638
Bore/stroke ratio
1.074
Construction
Block material
cast iron
Head material
cast iron
Valvetrain
OHV
Fuel type
gas
Firing order
1-8-4-3-6-5-7-2
Years
1979–1987
What these numbers mean
Bore/stroke character
1.074 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves
Mean piston speed at 6,000 RPM
3,480 ft/min — conservative for a street engine
Rod ratio
1.638 — moderate — the range most production V8s fall in
Displacement per cylinder
38.1 ci
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