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GMC 305A specs: 305 ci 6-cylinder, 4.25 x 3.58 in (108 x 90.9 mm) bore and stroke. Rated 150 hp, 1960–1961. Used in the GMC C/K Pickup.
The GMC 305A is oversquare — the 4.25" bore exceeds the 3.58" stroke by 18.7% — a layout that favors airflow and RPM over leverage at the crank. The 3.58" stroke puts mean piston speed at 4,000 ft/min around 6,700 RPM, which is where a production bottom end is generally considered at its limit. Each of the 6 cylinders displaces 50.8 ci (833 cc).
It is one of 6 engines in the GMC 305ci family, all of them 305 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated at 150 hp across 1960–1961.
| Displacement | 305 ci |
|---|---|
| Bore | 4.25" |
| Stroke | 3.58" |
| Cylinders | 6 |
| Configuration | V6 |
| Bore/stroke ratio | 1.187 |
| Fuel type | gas |
|---|---|
| Advertised HP | 150 hp |
| Years | 1960–1961 |
| Bore/stroke character | 1.187 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,580 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 50.8 ci |
Each link opens the calculator with this engine's specs already filled in.
These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1960 | 150 hp | 305A |
| 1961 | 150 hp | 305A |
1 factory vehicle, 1960–1961. Each year opens that model's engine table.