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American Motors 258ci specs: 258 ci (4.2L) 6-cylinder, 3.75 x 3.90 in (95.3 x 99.1 mm) bore and stroke, 8.67:1 compression. Rated 110 hp, 1979.
The American Motors 258ci 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).
8.67:1 is low by modern standards and generally happy on regular pump fuel.
It is one of 2 engines in the American Motors 258ci family, all of them 258 ci — the differences between them are in compression, heads and rating, not displacement.
| Displacement | 258 ci / 4.2 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3.9" |
| Cylinders | 6 |
| Compression ratio | 8.67:1 |
| Rod length | 5.875" |
| Rod ratio | 1.506 |
| Bore/stroke ratio | 0.962 |
| Piston deck clearance | 0.022" |
| Induction | 1-2 |
|---|---|
| Advertised HP | 110 hp |
| Years | 1979 |
| 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 | 8.67:1 — 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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