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Plymouth RB (1963–1971) 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, 1963–1971.
The Plymouth RB (1963–1971) 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. 12 factory variants share these dimensions, rated from 365 to 425 hp across 1963–1971.
| Displacement | 426 ci / 7 L |
|---|---|
| Bore | 4.25" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 10.2–11:1 (published figures vary) |
| Bore/stroke ratio | 1.133 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Firing order | 1-8-4-3-6-5-7-2 |
| Advertised HP | 365–425 hp |
| Years | 1963–1971 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. | Duration int / exh | Valve lift int / exh | LSA |
|---|---|---|---|---|---|---|---|
| 1963 | gross | 415 @ 5600 | 470 lb-ft @ 4400 | 11:1 | — | — | — |
| 1964 | gross | 365 @ 4800 | 470 lb-ft @ 3200 | 10.3:1 | — | — | — |
| 1964 | gross | 415 @ 5600 | 470 lb-ft @ 4400 | 11:1 | — | — | — |
| 1965 | gross | 365 @ 4800 | 470 lb-ft @ 3200 | 10.3:1 | 268° | 0.430" | 110° |
| 1967 | gross | 425 @ 5000 | 490 lb-ft @ 4000 | 10.25:1 | 276° | 0.467" / 0.473" | 112° |
| 1968 | gross | 425 @ 5000 | 490 lb-ft @ 4000 | 10.25:1 | 284° | 0.467" / 0.473" | 112° |
| 1969 | gross | 425 @ 5000 | 490 lb-ft @ 4000 | 10.25:1 | — | — | — |
| 1971 | gross | 425 @ 5000 | 490 lb-ft @ 4000 | 10.2:1 | — | — | — |
Factory specification sheet. Durations are the factory's advertised figures at the basis printed on the sheet, not duration at .050" lift; lobe separation is calculated from the printed valve timing.
| 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 |
|---|---|---|
| 1963 | 415 hp | — |
| 1963 | 425 hp | — |
| 1964 | 365 hp | — |
| 1964 | 415 hp | — |
| 1964 | 425 hp | — |
| 1965 | 365 hp | — |
| 1966 | 425 hp | — |
| 1967 | 425 hp | — |
| 1968 | 425 hp | — |
| 1969 | 425 hp | — |
| 1970 | 425 hp | — |
| 1971 | 425 hp | — |
5 factory vehicles, 1963–1971. Each year opens that model's engine table.