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Plymouth RB (1965–1971) specs: 426 ci (7.0L) 8-cylinder, 4.25 x 3.75 in (108 x 95.3 mm) bore and stroke. Rated 365-425 hp, 1965–1971.
The Plymouth RB (1965–1971) is oversquare — the 4.25" bore exceeds the 3.75" stroke by 13.3% — a layout that favors airflow and RPM over leverage at the crank. The 3.75" 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 53.3 ci (873 cc).
It is one of 16 engines in the RB family, sitting between the 413 ci and 440 ci members. 7 factory variants share these dimensions, rated from 365 to 425 hp across 1965–1971.
| Displacement | 426 ci / 7 L |
|---|---|
| Bore | 4.25" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.133 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 365–425 hp |
| Years | 1965–1971 |
| Bore/stroke character | 1.133 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,750 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 53.3 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 |
|---|---|---|
| 1965 | 365 hp | — |
| 1966 | 425 hp | — |
| 1967 | 425 hp | — |
| 1968 | 425 hp | — |
| 1969 | 425 hp | — |
| 1970 | 425 hp | — |
| 1971 | 425 hp | — |