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Jeep 258 specs: 258 ci (4.2L) 6-cylinder, 3.75 x 3.90 in (95.3 x 99.1 mm) bore and stroke, 9.2:1 compression. Rated 112 hp. OHV. Rod ratio and piston speed.
The Jeep AMC Straight-6 (258) 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. Each of the 6 cylinders displaces 43.0 ci (705 cc).
9.2:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 14 engines in the AMC Straight-6 family, the largest at 258 ci. Factory fitments include the Jeep CJ-7, Jeep Wrangler YJ, Jeep Cherokee SJ.
| Displacement | 258 ci / 4.2 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3.9" |
| Cylinders | 6 |
| Compression ratio | 9.2:1 |
| Bore/stroke ratio | 0.962 |
| Piston deck clearance | 0.022" |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 112 hp |
| 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 |
| Displacement per cylinder | 43.0 ci |
| Static compression | 9.2:1 — low by modern standards, generally happy on regular pump fuel |
| Chamber volume | 67.84 cc |
|---|---|
| Rocker ratio | 1.6:1 |
| Lifter type | hydraulic |
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