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Jeep 6-258 specs: 258 ci (4.2L) 6-cylinder, 3.75 x 3.895 in (95.3 x 98.9 mm) bore and stroke, 8:1 compression. Built 1972–1973. Used in the Jeep Jeepster.
The Jeep 6-258 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. A rod ratio of 1.508 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 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 15 engines in the AMC Straight-6 family, the largest at 258 ci. 2 factory variants share these dimensions across 1972–1973.
| Displacement | 258 ci / 4.2 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3.895" |
| Cylinders | 6 |
| Configuration | I6 |
| Compression ratio | 8:1 |
| Rod length | 5.875" |
| Rod ratio | 1.508 |
| Bore/stroke ratio | 0.963 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Firing order | 1-5-3-6-2-4 |
| Years | 1972–1973 |
| 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 |
| Rod ratio | 1.508 — 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 |
|---|---|---|
| 1972 | — | 6-258 |
| 1973 | — | 6-258 |
1 factory vehicle, 1972–1973. Each year opens that model's engine table.