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GMC 402 specs: 402 ci (6.6L) 8-cylinder, 4.125 x 3.76 in (104.8 x 95.5 mm) bore and stroke, 8.5:1 compression. Rated 240 hp. OHV. Rod ratio and piston speed.
The GMC Big Block Chevy (402) is oversquare — the 4.125" bore exceeds the 3.76" stroke by 9.7% — a layout that favors airflow and RPM over leverage at the crank. The 3.76" stroke puts mean piston speed at 4,000 ft/min around 6,400 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 50.3 ci (823 cc).
8.5:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 5 engines in the Big Block Chevy family, sitting between the 396 ci and 502 ci members. Factory fitments include the GMC C/K Series, GMC Suburban.
| Displacement | 402 ci / 6.6 L |
|---|---|
| Bore | 4.125" |
| Stroke | 3.76" |
| Cylinders | 8 |
| Compression ratio | 8.5:1 |
| Bore/stroke ratio | 1.097 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 240 hp |
| Bore/stroke character | 1.097 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,760 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 50.3 ci |
| Static compression | 8.5:1 — low by modern standards, generally happy on regular pump fuel |
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