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American Motors 258ci (1971–1983) specs: 258 ci (4.2L) 6-cylinder, 3.75 x 3.90 in (95.3 x 99.1 mm) bore and stroke, 8:1 compression. Rated 95-150 hp.
The American Motors 258ci (1971–1983) is close to square: 3.75" bore against 3.9" stroke, within 3.8%, which splits the difference between a high-revving layout and a torque layout. The 3.9" stroke puts mean piston speed at 4,000 ft/min around 6,150 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.506 is short — higher piston acceleration and more side-load, typical of stroker combinations and a reason to watch piston speed closely. Each of the 6 cylinders displaces 43.0 ci (705 cc).
8:1 is low by modern standards and generally happy on regular pump fuel.
It is one of 4 engines in the American Motors 258ci family, all of them 258 ci — the differences between them are in compression, heads and rating, not displacement. 23 factory variants share these dimensions, rated from 95 to 150 hp across 1971–1983.
| Displacement | 258 ci / 4.2 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3.9" |
| Cylinders | 6 |
| Configuration | I6 |
| Compression ratio | 8:1 |
| Rod length | 5.875" |
| Rod ratio | 1.506 |
| Bore/stroke ratio | 0.962 |
| Fuel type | gas |
|---|---|
| Firing order | 1-5-3-6-2-4 |
| Advertised HP | 95–150 hp |
| Years | 1971–1983 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1971 | gross | 150 @ 3800 | 240 lb-ft @ 1800 | 8:1 |
| 1973 | net | 110 @ 3500 | 195 lb-ft @ 2000 | 8:1 |
Factory specification sheet.
| Bore/stroke character | 0.962 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 3,900 ft/min — in the range typical of performance street engines |
| Rod ratio | 1.506 — short — more side loading and piston dwell at TDC, typical of stroker combinations |
| Displacement per cylinder | 43.0 ci |
| Static compression | 8:1 — low by modern standards, generally happy on regular pump fuel |
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 |
|---|---|---|
| 1971 | 150 hp | — |
| 1972 | 110 hp | — |
| 1973 | 110 hp | — |
| 1974 | 110 hp | — |
| 1975 | — | — |
| 1975 | 95 hp | — |
| 1976 | — | — |
| 1976 | 95 hp | — |
| 1976 | 120 hp | — |
| 1977 | — | — |
| 1977 | 95 hp | — |
| 1977 | 120 hp | — |
| 1978 | — | — |
| 1978 | 100 hp | — |
| 1978 | 120 hp | — |
| 1979 | — | — |
| 1979 | 100 hp | — |
| 1980 | — | — |
| 1980 | 110 hp | — |
| 1981 | — | — |
| 1981 | 110 hp | — |
| 1982 | — | — |
| 1983 | — | — |
9 factory vehicles, 1971–1983. Each year opens that model's engine table.