GMC 305ci Engine Specs

Bore, stroke, displacement and compression for all 6 GMC 305ci engines, compared side by side. Plus rod ratio and head specs where published.

At a glance

Displacement	305 ci
Cylinders	6, 8
Bore	3.736–4.250"
Stroke	3.484–3.580"
Compression	8.4–9.5:1
Advertised power	145–170 hp
Years	1960–1982

Engines in this family

Engine	Displacement	Bore × Stroke	CR	HP
GMC 305ci (1977–1978)	305 ci	3.736" × 3.484"	8.5:1	145
LG9	305 ci	3.736" × 3.484"	8.4:1	—
LE9	305 ci	3.736" × 3.484"	9.5:1	165
305A	305 ci	4.25" × 3.58"	—	150
305D	305 ci	4.25" × 3.58"	—	165
305E	305 ci	4.25" × 3.58"	—	165–170

Factory cars with a GMC 305ci

• GMC C/K Pickup: 1960, 150 hp; 1961, 150 hp; 1961, 165 hp; 1962, 165 hp; 1963, 165 hp; 1964, 165 hp; 1965, 165 hp; 1966; 1967, 170 hp; 1968, 170 hp; 1969, 170 hp

What varies across the family

Both bore and stroke vary across the family, so these are separate castings rather than variations on one block.

All are oversquare — bore larger than stroke, which favours revs over low-end torque.

Each engine page carries rod ratio, mean piston speed, bore/stroke character and cylinder head specs where published, and links into the calculators with its dimensions already filled in.

← All GMC engines