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Buick 425 specs: 425 ci (7.0L) 8-cylinder, 4.313 x 3.64 in (109.5 x 92.5 mm) bore and stroke, 10.25:1 compression. Rated 340 hp. Rod ratio and piston speed.
The Buick Nailhead V8 (425) is oversquare — the 4.3125" bore exceeds the 3.64" stroke by 18.5% — a layout that favors airflow and RPM over leverage at the crank. The 3.64" stroke puts mean piston speed at 4,000 ft/min around 6,600 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 53.1 ci (871 cc).
10.25:1 on iron heads is in the range that usually runs on pump premium; iron holds heat, so treat it as the less forgiving side of that range.
It is one of 2 engines in the Nailhead V8 family, the largest at 425 ci. Factory fitments include the Riviera, Wildcat.
| Displacement | 425 ci / 7 L |
|---|---|
| Bore | 4.3125" |
| Stroke | 3.64" |
| Cylinders | 8 |
| Compression ratio | 10.25:1 |
| Bore/stroke ratio | 1.185 |
| Piston deck clearance | 0.028" |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 340 hp |
| Bore/stroke character | 1.185 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,640 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 53.1 ci |
| Static compression | 10.25:1 — in the range that usually runs on pump premium |
| Intake valve diameter | 1.875" |
|---|---|
| Exhaust valve diameter | 1.5" |
| Chamber volume | 123.72 cc |
| Rocker ratio | 1.6:1 |
| Lifter type | hydraulic |
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