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GMC Power Pack specs: 265 ci (4.3L) 8-cylinder, 3.75 x 3.00 in (95.3 x 76.2 mm) bore and stroke. Rated 180-205 hp, 1955–1956. OHV. Rod ratio and piston speed.
The GMC Power Pack is oversquare — the 3.75" bore exceeds the 3" stroke by 25.0% — a layout that favors airflow and RPM over leverage at the crank. The 3" stroke does not reach 4,000 ft/min of mean piston speed until about 8,000 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 8 cylinders displaces 33.1 ci (543 cc).
It is one of 23 engines in the Small Block Chevy family, the smallest at 265 ci. 2 factory variants share these dimensions, rated from 180 to 205 hp across 1955–1956.
| Displacement | 265 ci / 4.3 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.25 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 180–205 hp |
| Years | 1955–1956 |
| Bore/stroke character | 1.250 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,000 ft/min — conservative for a street engine |
| Displacement per cylinder | 33.1 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 |
|---|---|---|
| 1955 | 180 hp | Power Pack |
| 1956 | 205 hp | Power Pack |