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Oldsmobile 257ci specs: 257 ci (4.2L) 6-cylinder, 3.531 x 4.375 in (89.7 x 111.1 mm) bore and stroke, 6.5:1 compression. Rated 105 hp, 1949.
The Oldsmobile 257ci is undersquare — the 4.375" stroke exceeds the 3.531" bore by 19.3% — a layout that trades peak RPM for leverage and torque low in the range. With a 4.375" stroke, mean piston speed reaches 4,000 ft/min at only 5,500 RPM — the practical ceiling for a production bottom end — which is why a stroke this long makes its power low and runs out of safe revs early. A rod ratio of 1.514 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 42.8 ci (702 cc).
6.5:1 is low by modern standards and generally happy on regular pump fuel.
| Displacement | 257 ci / 4.2 L |
|---|---|
| Bore | 3.531" |
| Stroke | 4.375" |
| Cylinders | 6 |
| Compression ratio | 6.5:1 |
| Rod length | 6.625" |
| Rod ratio | 1.514 |
| Bore/stroke ratio | 0.807 |
| Advertised HP | 105 hp |
|---|---|
| Years | 1949 |
| Bore/stroke character | 0.807 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 4,375 ft/min — in the range typical of performance street engines |
| Rod ratio | 1.514 — short — more side loading and piston dwell at TDC, typical of stroker combinations |
| Displacement per cylinder | 42.8 ci |
| Static compression | 6.5:1 — low by modern standards, generally happy on regular pump fuel |
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