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Plymouth RB (1961–1962) specs: 413 ci (6.8L) 8-cylinder, 4.188 x 3.75 in (106.4 x 95.3 mm) bore and stroke, 10.1:1 compression. Rated 350-420 hp, 1961–1962.
The Plymouth RB (1961–1962) is oversquare — the 4.1875" bore exceeds the 3.75" stroke by 11.7% — 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 51.6 ci (846 cc).
10.1:1 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 16 engines in the RB family, the smallest at 413 ci. 4 factory variants share these dimensions, rated from 350 to 420 hp across 1961–1962.
| Displacement | 413 ci / 6.8 L |
|---|---|
| Bore | 4.1875" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 10.1:1 |
| Bore/stroke ratio | 1.117 |
| 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 | 350–420 hp |
| Years | 1961–1962 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1961 | gross | 350 | — | 10.1:1 |
Factory specification sheet.
| Bore/stroke character | 1.117 — 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 | 51.6 ci |
| Static compression | 10.1:1 — in the range that usually runs on pump premium |
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 |
|---|---|---|
| 1961 | 350 hp | — |
| 1962 | 385 hp | — |
| 1962 | 410 hp | — |
| 1962 | 420 hp | — |
3 factory vehicles, 1961–1962. Each year opens that model's engine table.