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Jeep AMC Straight-6 (1972–1986) specs: 258 ci (4.2L) 6-cylinder, 3.75 x 3.895 in (95.3 x 98.9 mm) bore and stroke. Rated 110-115 hp, 1972–1986. OHV.
The Jeep AMC Straight-6 (1972–1986) is close to square: 3.75" bore against 3.895" stroke, within 3.7%, which splits the difference between a high-revving layout and a torque layout. The 3.895" 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. Each of the 6 cylinders displaces 43.0 ci (705 cc).
It is one of 14 engines in the AMC Straight-6 family, the largest at 258 ci. 9 factory variants share these dimensions, rated from 110 to 115 hp across 1972–1986.
| Displacement | 258 ci / 4.2 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3.895" |
| Cylinders | 6 |
| Configuration | I6 |
| Bore/stroke ratio | 0.963 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 110–115 hp |
| Years | 1972–1986 |
| Bore/stroke character | 0.963 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 3,895 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 43.0 ci |
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These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1972 | 110 hp | — |
| 1973 | 110 hp | — |
| 1980 | — | — |
| 1981 | — | — |
| 1982 | — | — |
| 1983 | — | — |
| 1984 | 115 hp | — |
| 1985 | 115 hp | — |
| 1986 | 115 hp | — |